Abstract During the second half of the eighteenth century, the study of volcanism was related to the question of orogenesis and the controversial lithogenesis of basalt. In the Italian peninsula, the key outcrops occurred mainly along the foothills of the Alps of Veneto. Moreover, the question of the origin of columnar basalts and other rocks (porphyry, granite) involved theories on the age of the Earth and the possible recognition of an evolutionary process of the making of lithosphere. Consequently, following explorations in the Alps and Prealps several scientists began to regard the basaltic formations as evidence of the relationships between mountains and ancient volcanoes. Nevertheless, especially from the 1780s, the spread of Wernerian theory in some Italian States led to criticism of the vulcanists' conclusions. Thus, some naturalists working in Lombardy tried to re-establish the idea of a great flood to explain the morphology of the Central Alps. Discussing this complex situation, the paper analyses the development of regional geology, based on fieldwork, which emphasized the function of volcanic activity in the history of building the Earth's crust and mountains.

Abstract The Town Creek locale in Jackson, Mississippi exposes fossiliferous strata through stream erosion and provides evidence for stratal doming atop an extinct volcano. Charles Lyell (1797–1875) investigated the locale on his second visit to North America (1845–46) and concluded that Town Creek fossils were older than Vicksburg fossils, and that strata dipped westward from Jackson. In the 1850s, Mississippi's state geologist Eugene Hilgard (1833–1916) recognized the first volcanic doming evidence and correctly concluded that a ‘local upheaval’ had elevated the Jackson area. Subsequent research revealed an extinct volcano as Jackson Dome's source, explaining the strata dip and Lyell's observation that Jackson's Eocene fossils were older than Vicksburg fossils, later identified as Oligocene. The Jackson fossiliferous strata, known today as the Moodys Branch Formation, exquisitely preserves fossils that have contributed to numerous scientific investigations. The Town Creek locale was threatened in 2003 when a proposed flood control project would have inundated it. Scientists rallied for the preservation of the geologically and historically important Town Creek, and today it remains as a geoheritage site available to scientists and the public, with a state historical marker noting its geological importance. Long-term preservation of the locale is needed but not guaranteed.

Abstract The Geitafell Volcano, an extinct Tertiary central volcano, was active in the Icelandic rift zone between 5 and 6 Ma. Because of deep glacial erosion, the interior of the volcano is exposed from its flanks to the top of the extinct crustal magma chamber represented by several gabbro plutons. Here we present the results of a detailed study of the infrastructure of the Geitafell Volcano. The study includes measurements of the shape, size and fracture pattern of the extinct crustal magma chamber and its remarkably sharp contact with a dense swarm of inclined sheets for which the chamber acted as a source. A comparison of the attitude of 1087 joints within the basaltic magma chamber (the pluton) with the attitude of 48 sheets cutting the chamber indicates that the injection of late-formed sheets from the still-molten inner part of the crustal magma chamber was structurally controlled by the cooling joints in the solidified outer part of the chamber. Similarly, the main trends of 408 mineral veins of the fossile geothermal system, ENE–WSW, NW–SE and NNE–SSW, coincide with those of cooling joints, suggesting that geothermal fluids circulated through the heat source of the geothermal system via cooling joints. Our study of 592 sheets from the dense swarm surrounding the crustal magma chamber suggests that most are inclined sheets whereas some are radial dykes. Their thickness distribution follows a negative exponential law with an average arithmetic thickness of 0.64 m. Most inclined sheets dip from 40 to 70° with the general dip decreasing with increasing distance from the magma chamber. The sheet swarm is generally bowl-shaped and concave upwards. Reconstruction of the magma chamber indicates that at its maximum size it was about 8 km in diameter, 1–2 km thick and with a top at 1.5 km depth below the surface. Numerical models using this magma-chamber geometry as a basis can formally account for the intensity and geometry of the sheet swarm of the Geitafell Volcano.