Howel Williams, known to all of his close friends as “Willie,” has inspired a generation of students and a host of professional colleagues through his enthusiastic devotion to the study of volcanoes. Willie, an identical twin of Welsh ancestry, was born October 12, 1898, in Liverpool, England. Toward the end of the First World War, he served with a survey unit of the Royal Engineers, and later, while resuming his interrupted course in geography at the University of Liverpool, his interest was kindled first in archeology and then in geology. According to legend, the fossilized trackway of a Triassic reptile imprinted on a large slab of local sandstone on display in the Geology Department fired his imagination and helped to lure him into the paths of geology. Clambering as a youth among the volcanic rocks of his parental county of Caernarvonshire in North Wales proved to be a prelude to his initial ventures into geological research, first on the igneous rocks of the Capel Curig district and then on the geology of the nearby mountainous tract of lower Paleozoic rocks centered on the peak of Snowdon. His classic account of The Geology of Snowdon (1927) was acclaimed especially for its lucid descriptions of the sequence, structure, and petrology of the Ordovician volcanic suite, and for the excellence of his mapping. It was in Snowdonia that he acquired an abiding love of field geology. This introduction to volcanic geology encouraged Willie to come west for three years as a Commonwealth Fellow

Williams, Howel, 1921, Excavations of Bronze Age Tumulus, near Gorsedd, Holywell, Flintshire: Archaeologia Cambrensis, 1921, p. 265–289. ——— 1922, Fish-bone from the Gorsedd Tumulus, Holywell: Archaeologia Cambrensis, 1922, p. 150–152. ——— 1922, The igneous rocks of the Capel-Curig district, North Wales: Liverpool Geol. Soc. Proc., v. 13, p. 166–202. Williams, Howel, and Williams, David, 1924, Easter excursion to Snodonia: Liverpool Geol. Soc. Proc., v. 14, p. 12–15. Williams, Howel, 1926, Notes on the characters and classification of pyroclastic rocks: Liverpool Geol. Soc. Proc., v. 14, p. 223–248. ——— 1927, The geology of Snowdon (North Wales): Geol. Soc. London Quart. Jour., v. 83, p. 346–427. ——— 1927, Kilauean ashes: Volcano Letter, no. 125. ——— 1928, A recent volcanic eruption near Lassen Peak, California: California Univ. Pub., Dept. Geol. Sci. Bull., v. 17, no. 7, p. 241—263. ———1929, Age of the Tahitian coral-reefs: Nature, v. 74, p. 727–728. ——— 1929, Geology of the Marysville Buttes, California: California Univ. Pub., Dept. Geol. Sci. Bull., v. 18, no. 5, p. 103-220. ——— 1929, The volcanic domes of Lassen Peak and vicinity, California: Am. Jour. Sci., 5th ser., v. 18, p. 313–330. Greenly, Edward, and Williams, Howel, 1930, Methods in geological surveying: London, Thomas Murby and Co., New York, D. Van Nostrand Co., 420 p. Williams, Howel, 1930, Notes on the later geologic history of Tahiti (Society Islands): California Univ. Pub., Dept. Geol. Sci. Bull., v. 19, no. 5, p. 119–135. ——— 1930, The Snowdon district—Report of the Easter field meeting, 1930:

Silicic volcanic rocks — dacite, rhyolite, and quartz porphyry — constitute about 35 percent of the Yavapai Supergroup, an older Precambrian sequence in central Arizona. In addition, the series contains about 30 percent pillow and amygdaloidal basalt, 5 percent andesitic rocks, and the remainder is mixed andesitic and silicic-bedded tuffaceous rock. The Yavapai Supergroup is divided into the Ash Creek and Alder Groups, each containing about 20,000 feet of lavas and pyroclastic rocks; no evidence is available to determine the relative ages of the two groups. The pillow basalts, coarsely graded volcanic breccias, and poorly sorted and graded tuffaceous-bedded rocks suggest a marine accumulation in an eugeosyncline. The Yavapai Supergroup is metamorphosed to the greenschist facies. The Ash Creek Group exhibits open folds but is nonfoliated except locally, whereas, the Alder Group is isoclinally folded and dominantly schistose. Twenty-five chemical analyses of the metamorphosed, silicic volcanic rocks reveal varying ratios of sodium and potassium. By comparison with young rhyolitic obsidians, only two rhyolites approach the ratios and total alkali content found in recent obsidians. Five rhyolites contain more sodium and appreciably less potassium, whereas none of the rhyolitic rocks contain more potassium than in young obsidians. High-sodium content is reflected by much albite, and high-potassium content by abundant sericite. Iron-magnesium metasomatism locally has produced much chlorite associated with an increased quartz and decreased albite content. The source of the iron and magnesium may be underlying pillow basalts. Many rhyolitic rocks of Phanerozoic age display variable ratios of Na 2 O and K 2 O caused by hydration of obsidian, hot spring action, and hydrothermal solutions. The redistribution of the alkalis in the Yavapai Supergroup may be caused in part by factors other than regional metamorphism. It is suggested the H 2 O + content, averaging between 5 and 6 percent by volume in these metamorphosed silicic rocks, may be derived in part from interstitial marine water.

Five Pliocene formations, all volcanic, overlie early Paleozoic (?) and Middle Triassic (?) sedimentary rocks and Cretaceous granitic rocks in the Bodie Hills, just north of Mono Lake, Mono County, eastern California. The Pliocene rocks include flows, dikes, plugs, and domes of dacite; andesite, basalt, latite, rhyolite, and rhyodacite flows; thick deposits of pyroclastic rocks include tuff, tuff breccia, welded tuff, and breccia flow. Oldest to youngest, the Pliocene formations are (1) Ranchería Tuff Breccia (revised name), mainly dacitic; (2) Murphy Spring Tuff Breccia (new name), mainly dacitic pyroclastic rocks, but with dacite and rhyolite flows; (3) Willow Springs Formation (revised name), dacite flows and tuff breccias, rhyodacite and rhyolite flows; (4) Mt. Biedeman Formation (revised name), rhyolite to basalt flows and tuff breccia; and (5) Potato Peak Formation (revised name), mainly andesite flows. Two structural features dominate the area: (1) the newly described Big Alkali caldera (this paper), a small circular collapse structure, which is marked by topographic expression and a gravity low, and (2) the Mt. Beideman dome, a volcanic pile built of basaltic and andesitic pyroclastic rocks followed by rhyolite, dacite, and andesite flows, plugs, and domes.

The Circle Creek Rhyolite is a multiple-source mass of fayalite-ferro-augite rhyolite, forming an extensive complex in northern Elko County, Nevada, with a diameter of about 8 miles and an exposed area of about 36 square miles. From the structure of the flow layering and the complex sequence of microbrecciation structures, it seems to be a relatively thick mass that rose passively through many fissures to flood a sag basin in the older rocks, which are as young as the early Pliocene Idavada Volcanics, a sequence of ignimbrites and tuffs. Chemical analyses, thin-sections, and X-ray diffractometer studies show that the Circle Creek Rhyolite is a two-feldspar rhyolite. The interrelations of the pyroxene phases are unusually complex. The Circle Creek is partly concealed by later tuffs, gravels, and olivine basalt and is cut by two sequences of high-angle faults of different ages.

Cenozoic rocks include about 15,000 feet of continental sedimentary and volcanic rocks of Oligocene age. Volcanic rocks consist of ignimbrite flows, plugs and dikes, lavas, and air-fall tuffs. Ignimbrite plugs and dikes fill vents from which some of the surrounding ignimbrite sheets were probably discharged. Plug rocks have vitroclastic textures and textures transitional between vitroclastic and flow banding. Dike rocks have vitroclastic textures and show various stages of vesiculation. Ignimbrites forming the basal sheet are crystal-rich rhyodacites, and have less silica and alkalies than the rhyodacites forming the top sheet. The ignimbrite plugs are more siliceous and alkalic than their surface-equivalent ignimbrite sheets. These plugs are modally and chemically “stratified”; their upper parts are crystal-rich rhyodacites and contain less silica and alkalies than the basal, moderately crystal-rich rhyolites. The origin of these variations is not well understood, but available data suggest that the basal portion of a differentiating magma chamber may have been tapped, so that the magma became progressively more siliceous and alkalic; continued pulses of increasingly more siliceous material may account for the “stratification” of the plugs. Cognate crystals in the ignimbrite plugs, dikes, and sheets were broken prior to extrusion; some were broken during compaction and welding. Most of the vesiculation took place prior to extrusion. Fluidization of the constituents may account for the mobility of the ignimbrites. Chemical compositions and textures have been modified in most sheets after they were emplaced. Secondary sanidine of a devitrification origin locally crystallized to a more sodic sanidine, and the replaced potassium may be accounted for by the K-feldspar rims on cognate crystals. Textural changes include welding, devitrification, and vapor-phase crystallization. The ignimbrite plugs and thick ignimbrite sheets are welded and devitrified from base to top. Thin sheets are welded only in their middle portions, and devitrification is confined to their upper zones. Ignimbrite plugs and their equivalent ignimbrite sheets contain vapor-phase minerals from base to top in vesicles that formed after welding. These ignimbrites were probably richer in volatiles than those in which vapor-phase crystallization was confined to the upper zones.

Nine principal layers of tephra have been distinguished in the vicinity of Mount Katmai and Novarupta. These were products of the 1912 eruption, and five of them represent major explosive events. Isopachous maps for four of the five layers show conclusively that they originated at Novarupta; while the thickness and coarseness of the other — the bottom layer — suggest that it could not have come from Mount Katmai, and that Novarupta is very probably its source also. The great tuff flow in the Valley of Ten Thousand Smokes may have started before the explosive phase of the 1912 eruption, but a major part of it was emplaced after the first cataclysmic eruption of rhyolite tephra. The banded pumices of the tephra deposits indicate the mixing of two magmas — rhyolitic and andesitic. The rhyolitic chamber underlies Novarupta, while the andesitic chamber underlies the chain of Mounts Katmai, Trident, Mageik, and Martin. Only the topmost layer of very fine tephra originated from the conduit at Mount Katmai. Its ejection was accompanied by the outpouring of the Katmai River tuff flow from a source within the crater and by the outpouring of a smaller, but similar, tuff flow down the western slope of Mount Katmai over glacier 3. A small steeply dipping welded tuff of the 1912 eruption occurs within the crater along the western wall. Its presence there, together with the outpouring of tuff flows down the flanks of the volcano at a very late stage of the eruptive activity, indicate that the summit of Mount Katmai collapsed slowly during the entire period of the 1912 eruption and that the crater only reached its final dimensions after almost all activity ceased. The summit of Mount Katmai was supported during most of the eruption by a column of magma, whose upper part must have been at or close to the surface, but which did not erupt until all activity ceased at the principal source, Novarupta. This suggests that the two conduits were intimately connected throughout most of the 1912 eruption, but were finally separated near the end. The sequence of rhyolite, mixed magma, and rhyolite from the conduit at Novarupta also supports this hypothesis. The 2.6 cubic miles of tuff flow in the Valley of Ten Thousand Smokes reduces to 1.5 cubic miles of solid andesite, almost exactly equal to the volume of the crater and missing summit of Mount Katmai. This equivalence is probably fortuitous, however, because there is no apparent compensation anywhere for much of the 4.75 cubic miles of tephra (approximately 1.9 cubic miles of solid rock).

Paleomagnetic and potassium-argon studies support geologic evidence that the lower member of the Bandelier Tuff was deposited 1.4 m.y. ago. The upper member erupted about 1.0 m.y. ago and was followed by caldera collapse which formed the 12- to 14-mile diameter Valles Caldera. Postcaldera activity which resulted in the eruption of rhyolite domes and pyroclastic material, has occurred at about 0.9, 0.7, 0.5, and 0.4 m.y. ago, with later undated eruptions that were estimated at about 0.1 m.y. ago. These data from the Valles Caldera are the basis for the previously published age revision of the Brunhes-Matuyama geomagnetic polarity epoch boundary from 1.0 to 0.7 m.y. ago, and they were used to define the Jaramillo normal polarity event at about 0.9 m.y. ago (Doell and Dalrymple, 1966).

Examination of the relationships between textural variations and structural features in a complex, leucocratic dike swarm that is enclosed in the Half Dome quartz monzonite of the Sierra Nevada batholith led to the conclusion that alaskites, aplites, and pegmatites were produced by a continuous process, in which the degree of saturation of the dike fluid with water played a decisive role. The process began with segregation of the interstitial fluid of crystallizing quartz-monzonite magma by means of crystal settling and differential flow and left zones of mafic schlieren as the complementary residuum. It continued with emplacement of this leucocratic magma into fractures in more completely consolidated portions of the quartz monzonite, and ended with the formation of aplites and pegmatites. Important ramifications of this hypothesis are: (1) The dikes of upper Tenaya Canyon were emplaced by injection. (2) During the initial stages of dike formation, the magma was unsaturated with water, and leucocratic dike material crystallized as granitic-textured alaskite, where the fluid remained unsaturated during most of its crystallization. (3) Aplites resulted where leucocratic dike fluid became saturated with water, so that a small local decrease in pressure was capable of producing supersaturation, and the consequent separation of an aqueous gas phase. After a time interval during which the remaining dense, silicate-rich liquid persisted as a supercooled liquid, rapid nucleation at many centers produced aplite. (4) Granular pegmatite crystallized from the mobile, aqueous gas phase, after its accumulation in zones within the denser, supercooled liquid. (5) A very fine-grained felsitic aplite also developed locally from the gas phase, as the result of a “pressure quench” where increments of the gas phase were able to escape from the system.

Mono Basin is a post-Miocene, northeast-trending structural depression lying immediately east of the central Sierra Nevada. A negative gravity anomaly of about 50 milligals centers under Mono Lake in the western portion of the basin, which is surrounded by varied and voluminous volcanic rocks of Pliocene, Pleistocene, and Holocene age, and has been regarded by others as a volcano-tectonic depression. K-Ar ages and the distribution and relations of the young volcanic rocks show that they represent several separate volcanic episodes and that volcanism was synchronous around the basin only during the period between about 2 and 4 m.y. ago. Structural development of the present depression has occurred largely during the last 3 m.y. and is still in progress. Structures around the margins of Mono Basin suggest that it is a broad, probably shallow, warp with an axis that plunges gently toward the southwest where it terminates against the large dip-slip fault along the eastern front of the Sierra Nevada. The throw on this fault has been about 6000 feet, and the escarpment produced is now about half buried by basin fill. The gravity anomaly is probably produced by very low-density pumiceous fill rather than by extreme depth. The northeasterly trend of the basin is a reflection of a regional zone of northeasterly structural trends, extending far east of Mono Basin itself, within which left-lateral movements have occurred. The junction of this zone with northwest-trending structures to the south and west produces a structural “knee,” a zone of extension along which voluminous Pliocene-Pleistocene volcanic eruptions have occurred east of Mono Basin. The deepest part of the basin, under Mono Lake, is at the western end of this structural “knee.” Mono Basin is not a caldera-like collapse structure due to eruption of magma. The total estimated volume of lava erupted during Pliocene and Pleistocene time in the region around the basin has been only about one quarter of the volume of the present depression. Volcanism and deformation have been synchronous, and their manifestations are interrelated, but the relation appears to be incidental rather than necessary. Deformation appears to have been approximately continuous during the last 3 to 4 m.y. Rates of differential vertical movements in the marginal regions are estimated at 1 to 2 feet per thousand years, and between the margins and center of the basin to have been between 2 and 4 feet per thousand years.

Tephra deposits of the Honolulu Group on Oahu are middle to late Pleistocene and range from alkali basalt to melilite nephelinite. Some of the deposits are fresh, but most of them are zeolitic palagonite tuff. Where the most complete thicknesses of pyroclastic deposits are preserved, they are generally zoned from clayey soil 3 to 8 feet thick, down into relatively fresh tuff from 15 to 40 feet thick, which is underlain by palagonite tuffs as much as, or more than, 60 feet thick. The contact between fresh and palagonite tuff of Koko Crater roughly parallels the present deeply gullied land surface and indicates that the palagonite was formed long after the cone. The amount of authigenic minerals in tuffs is generally proportional to the amount of palagonite and indicates that minerals are related to the palagonitization process. The principal authigenic minerals were deposited in the following sequence, from first to last: phillipsite, chabazite, thomsonite, gonnardite, natrolite, analcime, montmorillonite together with opal, and calcite. Chemical analyses of sideromelane and associated palagonite by the electron microprobe show that about a third of the SiO2, half of the A12O3, and three quarters or more of the CaO, Na 2 O, and K 2 O are lost when sideromelane is converted to an equal volume of palagonite. A substantial proportion of these components lost from the sideromelane are precipitated nearby as zeolite, montmorillonite, opal, or calcite cement. Reaction of sideromelane with cold percolating ground water accounts for the vertical zoning from a surface layer of relatively fresh tuffs down into palagonite tuffs. The p H and ionic strength of percolating water probably increases with depth by solution and hydrolysis of glass, and where the p H and ionic strength become sufficiently high, the glass reacts to form palagonite and zeolites. A high p H probably accounts for the mobility of aluminum as reflected in its loss from palagonitized glass and its precipitation in zeolites. As support for a p H control of aluminum mobility, aluminum has remained immobile in palagonitic alteration of sideromelane pumice of the Pahala Ash on Hawaii. This pumice is presently weathering to palagonite in the soil profile, which contains water having a p H of 5 to 6. The sharp interface between unaltered sideromelane and palagonite suggests that palagonite was formed by a microsolution-precipitation mechanism rather than by simple hydration and devitrification. Rainfall, grain size, permeability, and original composition are important factors in determining the degree of palagonitic alteration, nature of the authigenic mineral assemblages, and crystal habit of phillipsite on Oahu.

Geologic mapping, paleomagnetic stratigraphy, and potassium-argon dating were used to determine the time and volume relations of tholeiitic and alkalic basalt on Nunivak Island in the Bering Sea near the coast of Alaska. Volcanism on Nunivak Island occurred in distinct episodes separated by quiet intervals that lasted from 1.6 to 0.6 m.y. During the past 6 m.y., tholeiitic basalt was erupted during at least five such episodes, and highly undersaturated alkalic basalt was erupted during at least three episodes. The oldest volcanic rock found on Nunivak Island is an alkalic basalt, succeeded by repeated alternations of tholeiitic and alkalic basalt. During the last episode of tholeiitic volcanism, 130 cu km of basalt erupted during a well-defined interval that lasted from 0.9 to 0.3 m.y. ago. A nearly contemporaneous eruption of alkalic basalt has continued vigorously to historic times and has covered the central part of the island with small cones, flows, and tephra from explosion craters. The volume of alkalic basalt of the latest episode of volcanic activity is from 0.7 to 2.0 percent of the volume of the associated tholeiite. At least one earlier eruption of alkalic basalt occurred in close association with the eruption of tholeiite. As on Hawaii, the highly undersaturated alkalic basalts on Nunivak contain abundant ultramafic inclusions.

The Southern Coulee is the southernmost and largest of the four Recent pumiceous rhyolitic coulees, or stubby flows, of the Mono Craters, eastern California. It is one of the youngest volcanic deposits of the Mono Craters and is largely bare and uneroded. The coulee is 3.6 km long and averages 1.2 km in width and 75 m in thickness. It was protruded from a north-trending fissure beneath the crest of the Mono Craters ridge. About two-thirds of the lava flowed west and one-third flowed east. The coulee has three main parts: the dome, located over the orifice, where flow was about vertical; the flow, where movement was lateral; and the talus slope, which surrounds the coulee and formed from the advancing steep-flow front. In addition, three small areas of air-fall pumice ash occur on the coulee and appear to be remnants of an ash eruption that took place during an early phase of the coulee eruption. Three distinctive lithologic units based on rock density related to the degree of vesicularity have been mapped: a unit of lowest density (average ρ = 0.65); a unit of intermediate density (average ρ = 1.20); and a unit of highest density (average ρ = 1.75). The contacts between the units are abrupt despite the fact that core drilling has shown the coulee to be a jumbled mass of blocks down to a depth of at least 45 m. The two less dense units, which consist of highly inflated thick-bedded pumice, form two connecting boat-shaped bodies along the entire south margin of the coulee. These units are probably not over 25 m thick and are underlain by the unit of highest density, which appears to form the rest of coulee. The dense unit consists of thin- to medium-bedded dense pumice and lesser amounts of obsidian. The above distribution of the lithologic units in the coulee was probably caused by the eruption of rocks characteristic of all the units from, the southern part of the north-trending fissure, while only rocks of the unit of highest density erupted from the northern part. The protrusion of the coulee involved several subordinate and, in places, interfering streams that had slightly different courses and levels. This complex flow resulted in a modification or disruption of the original spatial relations of the lithologic units, as they erupted from the orifice. The petrographic and chemical data indicate a uniform composition for the lava that belies its heterogeneous aspect. The lava is composed almost entirely of clear glass (average n D = 1.488 ± 0.001), and contains only trace amounts of microlites and cristobalite-sanidine spherulites. Eight chemical anlyses show a silica range from 74.7 to 76.2 percent and indicate a rhyolite composition of the sodipotassic subrang. This composition is characteristic of glassy-fluidal rhyolites.

The Tuscan Formation consists of dominant tuff breccia and lapilli tuff, and minor lava flows, flow breccias, and tuff; volcanic conglomerate, sandstone, and siltstone are important constituents in its western portion near and in the subsurface of the Sacramento Valley. Chemical analyses from the major source areas suggest that breccia clasts of the Tuscan Formation are andesite and basaltic andesite. The formation has a maximum exposed thickness of 1700 feet and once covered about 2000 square miles. Silicic ash-flow tuffs, included in the Tuscan Formation, previously have been correlated with the Nomlaki Tuff Member, but as several silicic tuff units are present, such correlations are not always justified. The late Pliocene age of the Tuscan Formation is supported by nonmarine diatoms. Radiometric dates and the almost total absence of paleosols in the Tuscan Formation suggest that its lahars were emplaced relatively rapidly, probably in less than a million years. Maps of the distribution and thickness of the Tuscan Formation are presented, from which it is inferred that laharic debris originally amounted to 300 cubic miles. Principal source areas of the Tuscan lahars include two eroded composite volcanoes south of Lassen Peak and two lesser source areas of indefinite form northwest of Lassen Peak. Small groups of tuff-breccia dikes west of the volcanoes apparently contributed only slight amounts of debris. Mount Yana, the chief source of the Tuscan and the southernmost volcano of the Cascade Range, was constructed chiefly of flows of pyroxene andesite and interbedded thick laharic units. A dike swarm that included many tuff-breccia dikes invaded the central part of the volcano. After volcanism ceased, slight fault movement disrupted the central part of the volcano and facilitated erosion, which then excavated a large central depression. Structural control was exerted by a major east-west lineament that probably marks the structural northern limit of the Sierra Nevada. Indirect evidence suggests vertical movement of about 2500 feet in Pliocene time. Water needed for the mobility of the Tuscan lahars probably did not come from extensive fields of snow or ice. Heavy rainfall is a possible source of water, but estimates of available magmatic and meteoric water show that ample water is available from these sources. Significant proportions of the lahars formed by near-surface autobrecciation in dikes and central conduits at temperatures less than 800° C, and probably in the range 340° to 280° C. The mechanism of brecciation proposed by Curtis (1954) probably was effective at Mount Yana.

New chemical analyses for major elements in 76 Hawaiian rocks are presented and bring the total number of such modern analyses to about 470. Many determinations of minor elements also are becoming available. Hawaiian petrology is discussed against this total background. The three major rock suites, tholeiitic, alkalic, and nephelinic, are chemically intergradational. The main mass of the volcanoes is tholeiitic, followed by a relatively small volume (generally less than 1 percent) of alkalic lavas; the two types of lavas are interbedded in a thin transitional zone. The nephelinic lavas are separated from the others by a long time interval that is marked by a profound erosional unconformity. Variations within the rock suites are largely the result of crystal differentiation. All three rock suites probably are derived from a single type of parent magma, which varies slightly from one volcanic center to another, of olivine tholeiite composition. Crystallization of this magma in shallow magma chambers leads to eruptible magmas of tholeiitic composition. In the last stages of volcanism, consolidation of the upper part of the magma body leads to crystallization at deeper levels under higher pressure and to production of alkalic magmas. Finally, crystallization at depths of several tens of kilometers produces nephelinic magmas that are erupted after a long period of volcanic quiescence.

The rocks of Tahiti constitute what is probably the most complete suite of strongly alkaline rocks in the Pacific Ocean. New analytical and petrographic data show that the differentiated rocks form two divergent series. Each series is represented by a wide range of effusive and plutonic rocks that differ mainly in their relative degree of silica enrichment. The end members of one series are trachytes and syenites that are almost exactly saturated; the second series trends toward phonolites and nepheline syenites. Fractionation of the principal minerals of the plutonic suite (feldspars, silica-poor pyroxenes, kaersutite, biotite, and iron-titanium oxides) should have caused strong enrichment of silica in residual liquids and could not have been the sole controlling factor in differentiation. The composition of crystalline phases seems to have been a response to magmatic differentiation rather than its cause. The compositions of the differentiated liquids can be explained as the products of differentiation in a vertically spreading zone of fusion in the upper mantle.

It is well known that the Boulder batholith region experienced intensive plutonism, volcanism, and tectonism that all began in Late Cretaceous time, after at least 700 m.y. of structural and igneous inactivity except for sporadic epeirogeny. Recent stratigraphic, structural, paleontologic, arid, especially, radiometric evidence makes it possible to date these dynamic events rather closely. The time relations that are revealed do not form a simple sequence of volcanism-folding-thrusting-batholith emplacement, as has often been supposed, but involve an intertwined complex. Significant volcanism began ~ 85 m.y. ago in late Coniacian or early San-tonian time, with deposition of the thick, local tuffaceous Slim Sam Formation. Volcanism climaxed from 77 to 79 m.y. ago, in early Campanian time, when the region was buried under at least 10,000 feet of calc-alkalic volcanic and volcaniclastic rocks, which included many sheets of welded tuff — the Elkhorn Mountains Volcanics —, and a vast amount of contemporaneous ash was airborne beyond the region. Major volcanism ceased ~ 73 m.y. ago, late in the Campanian, not to recur until early Eocene time, ~ 50 m.y. ago. The bulk of the batholith was emplaced beneath and within the volcanic edifice in early and middle Campanian time, during a 6 m.y. span from 78 to 72 m.y. ago, and some leucocratic masses were intruded during the next few million years, so that the whole batholith was emplaced within about 10 m.y. Folding at and near the site of the batholith began in late Coniacian or Santonian time and culminated before middle Campanian time; the main folding north and east of the batholith was post-Campanian, probably Maestrichtian. Thrusting began before middle Santonian time, and recurred intermittently well into the Maestrichtian, or even a little later. Thus volcanism, plutonism, folding, and thrusting began and ended within a few million years of each other, during the last 20 m.y. of the Cretaceous. Major folding, thrusting, and volcanism started about the same time, though not always at the same places, and a little earlier than plutonism. In any given locality, volcanism ended before major folding; the climax of plutonism followed the climax of volcanism; thrusting preceded and accompanied plu-tonism near the batholith, but followed plutonism farther away; thrusting ended a little later than folding. These dynamic processes so closely related in time and space must also be genetically related in the Boulder batholith region. Gilluly’s (1965) conclusion that the orogeny which produced the great Cretaceous thrusts of Montana was “essentially without plutonic associations” is not tenable.