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Published major-element analyses of Ethiopian volcanic rocks have been subjected to a systematic discriminant analysis. The plateau regions can be subdivided according to the proportions of alkaline and tholeiitic basalts. In northern Ethiopia, these subprovinces show increasing basalt alkalinity with time. The most voluminous basalts have lowest magnesium values, independent of the degree of alkalinity. Rift and Afar basalt chemistry falls within the spectrum observed for the plateau basalts, with no perceptible difference resulting from lithospheric attenuation beneath Afar. However, silicic volcanics of the Rift-Afar floor differ in bulk terms from those of the plateau margins in showing a stronger bias towards peralkalinity, and having higher Na/K values. Two particularly voluminous volcanic episodes have occurred in Ethiopia, dated at ?30–19 m.y. and 4.5–0 m.y. and which link well with one model for seafloor spreading in the Red Sea and Gulf of Aden. Evidence for a mantle hotspot under Ethiopia remains ambiguous.

New K-Ar dates on Ischia volcanic rocks are reported in order to reconstruct the temporal evolution of volcanic activity and to estimate the rate of associated energy release. The most probable age of eruption of the « green tuff » pumice flows forming the horst in the central part of the island is 0.74 (±0.09) m.y. The formation of the horst occurred in the time span 0.7–0.3 m.y. The subsequent volcanic activity was episodic. The oldest formations (0.37–0.31 m.y.) were formed by lava lake activity with small energy content, whereas formations younger than 0.1 m.y. were formed by eruptions characterized by higher explosivity and energetic content. No ages in the interval 0.31 (±0.02) −0.10 (±0.07) have been found. Geological considerations also suggest that volcanic activity was very weak or absent in Ischia during this time interval. The different energies and characteristics of the eruption at Ischia in relation to the tectonic pattern are used to evaluate the volcanic risk for the island.

Acid rocks make up about 10% of the whole Tertiary volcanic pile in eastern Iceland, or some 800 km1 of rocks between sea level and the summit level. Approximately half are extrusive flows, which range in form from steep-sided domes to tabular flows and have individual volumes of up to 2.5 km1. Most of the other half consists of acid pyroclastic rocks, in which ignimbrites (pyroclastic flow deposits) and air-fall tuffs or agglomerates are equally abundant. Composite lavas and mix-lavas, in which basic and acid magmas were erupted together, are common and so are the corresponding minor intrusions and pyroclastic rocks.

We have now reviewed in the briefest fashion the main facts relating to Scottish Carboniferous and Permian volcanicity. For a more detailed summary the reader is referred to two recently published Geological Survey handbooks of the British Regional Geology series (see ≪References≫, below). One of these, dealing with the Midland Valley of Scotland, gives petrological details regarding the most important and most intensively studied volcanic district. The other summarizes our less extensive knowledge of the volcanicity of the south of Scotland (Southern Uplands). Both volumes contain comprehensive bibliographies. Information regarding the Carboniferous and Permian rocks of Arran is best obtained from the Geological Survey memoir on that island. The Carboniferous volcanic rocks of Kintyre are dealt with in two papers referred to below. The Arran and Kintyre references are of purely local interest, but are included for the sake of completeness; the localities concerned are not dealt with ia the two Regional handbooks. A reference is also given below to a paper byJ. S. Turner. This useful summary of Carboniferous volcanicity in north-western and central Europe deals with the igneous rocks of (i) the Variscan Geosyncline and (ii) the Variscan Foreland, including Scotland. Up-to-date exhibits comprising specimens, maps, diagrams, and photographs, and specially designed to illustrate the geology and volcanicity of the areas described in the two Regional handbooks, are to be seen in the new Museum of Practical Geology opened in London in 1935. Numerous maps, models, and specimens, illustrative of Scottish volcanicity, are also displayed in the Royal Scotish Museum, Edinburgh. A large scale model of Arthur’rs Seat volcano is a noteworthy exhibit.

The Superior volcanic field occupies approximately 8,000 square kilometers of central Arizona in the zone between the southern Basin and Range Province and the Colorado Plateaus Province. The primary structural elements of an eruptive center in the western part of this field are: 1) volcanic plateau, 2) ring fracture zone, and 3) resurgent caldera core. A northwest trending graben controls the location of three small subsided blocks, the Willow Springs cauldron (2 km diameter), the Black Mesa cauldron (4 km diameter), and the Florence Junction cauldron (8 km diameter), which were centers for rhyolite ash and lava eruption. These late features are superimposed on a much larger volcano-tectonic structure, the Superstition resurgent cauldron which subsided at an earlier stage following the extrusion of quartz latite welded tuff.

Three major cycles of volcanism during the Miocene and Pliocene formed a layered succession of calc-alkaline eruptive materials in the western San Juan Mountains nearly 1.5 miles thick and having a volume greater than 1,000 cubic miles. Each cycle was characterised by major eruptions followed by subsidence in the vent areas, and the resulting structure was a great volcanic plateau surrounding a complex of nested cauldrons. In the first cycle, cruption of several hundred cubic miles of tuff breccia and subordinate lavas was followed by subsidence that created the San Juan volcanic depression, about 15 miles wide and 30 miles long. During the second cycle, pyroclastic rocks and lava flows accumulated within this depression and on its borders, and the depression subsided further. During the third cycle, ash flows spead widely from centres within the depression, and their eruption resulted in formation and subsidence of the nearly circular comagmatic Silverton and Lake City cauldrons, each about 10 miles across, within the earlier depression.