The fissure eruptions of Miocene basalt in the Columbia River region and the even more voluminous Eocene basalts of the Olympic Mountains—Oregon Coast Range occupy two different tectonic settings. Each of these great piles of “tholeiitic” lavas shows almost no differentiation, and rocks of intermediate (andesitic) composition are generally absent. In the adjacent “first cycle” Cascade Mountains, about 75 per cent of the volcanic rocks are andesites, though they range from olivine basalts to rhyolites. Violent steam explosions characterized their extrusion. The rocks are notably higher in CaO and Al 2 O 3 than those of the adjacent basaltic provinces. Rejuvenated or “second cycle” mountains,—the Rocky Mountains of Colorado—show a highly diversified volcanic sequence, including rhyolites, trachytes, latites, andesites, basalts, and also feldspathoidal lavas. Larsen’s studies of the San Juan area prove the coexistence and comingling of magmas of widely different compositions in spongy subterranean chambers. Tertiary igneous sequences occur in the elevated continental plates on both sides of the Rockies. The laccolithic complexes of the Colorado Plateau and the diorite-syenite-shonkinite (or lamprophyre) complexes found east of the Rockies resemble the San Juan sequence but are more alkalic. Still more alkalic are the highly feldspathoidal and ferromagnesian extrusives of the Navajo-Hopi country, which in their general features resemble the highly alkalic and ultramafic volcanic rocks of Uganda. Such alkalic mafic lavas commonly appear in small volume on the Precambrian shields. Common associates are intrusive complexes of alnoite, kimberlite, and mica peridotite. Also present on some shields are voluminous piles of tholeiitic basalt. The igneous cycle has ended; a new one begins. The widely held theory that all volcanic rocks originate by differentiation of basalt does not seem to fit this complex picture. The following hypothesis, admittedly oversimplified, is tentatively offered: Tholeiitic magma arises in a deep universal earth shell, but the fissure eruptions do not tap a great molten pool undergoing crystal differentiation. Differentiation occurs only after irruption to high levels. When a submarine trough is downbuckled into a tectogene, water plus other easily removable constituents are distilled from the metamorphosing geosynclinal sediments. These mix with the primitive tholeiite, modifying it to highly explosive steam-rich andesitic magma. Continued metamorphism of the tectogene root ultimately results in partial melting, causing the rise of trondjhemitic and granodioritic batholiths which may invade the slightly older extrusive andesites. Successive periods of anatexis will sweat upward more and more of the granitic fractions of the tectogene. Left behind in the dwindling root, the refractory greenstones, limestones, and serpentines have meanwhile been dehydrated by increasing metamorphism to amphibolites, skarns, and uralitized or biotized ultramafics. Locally these refractory hornblende- and biotite-rich rocks may be partially melted and erupted to produce the highly alkalic mafic lavas that characterize shields and continental plates.

Abstract The Appalachian mountain system of eastern North America comprises a complex of deformed rocks which has stimulated geologic thinking since the area was first settled in the 17th century. Little attention was paid to it in the early days of European settlement, but in the early nineteenth century interest grew rapidly, leading to the creation of numerous state surveys and research that has continued unabated to this day. The earliest tectonic concepts were understandably rather primitive and mostly concerned the simpler foreland portion of the orogen. Upheavals, explosions, and other vertical motions were the main ingredients of the theory of the Rogers brothers and the geosyncline of Hall and Dana. Awareness of the importance of horizontal movements increased as continued detailed mapping revealed the presence of large thrust faults, culminating in the middle of this century with the elucidation of a décollement tectonics that pervades the entire Appalachian foreland. The Appalachian crystallines were originally considered to be part of the crust, and they formed a large part of Appalachia, Dana’s geanticline that supplied sediment to the adjoining geosyncline. Studies early in the 1900s began to reveal the complexities of this terrane and by the middle of this century, the crystallines had been divided into a number of distinctive belts. With the application of the plate tectonics scheme in the 1970s, geologists realized that these belts probably are micro-continents, either broken off from the Laurentian craton, or having come from other plates. With the accession of deep seismic data, the crystallines are no longer thought of as being rooted in the crust, but are believed to have undergone an allochthony at least as extensive as the foreland’s.

The mountainous southwestern portion of Antigua consists mostly of calcalkaline igneous rocks, ranging in composition from quartz basalt to dacite, with andesite being most common. These are associated with agglomerates, tuffs, and a few limestone lenses which have been dated as mid-Oligocene. A small area of quartz diorite suggests that the center of volcanic activity was just to the south of the island and that a considerable thickness of pyroclastics has been eroded from the southern part of the island to expose the roots of the volcanoes which were active in mid-Oligocene time. The relation of the geologic events in the Lesser Antilles to the present theories of ocean-floor spreading is discussed briefly.