542 Geological Society : — 



scale ; and it is only by a careful study of the records of these two 

 great periods of igneous activity that we can hope to understand 

 the remarkable relations of the fragments of the intermediate 

 sedimentary formations, or to account for the peculiarities which 

 they present. 



That the rocks forming the great plateaux of the Hebrides and 

 the north of Ireland are really the vestiges of innumerable lava- 

 streams, is a fact which has long been recognized by geologists. 

 That these lavas were of subaerial and not subaqueous origin, is 

 proved by the absence of all contemporaneous interbedded sedi- 

 mentary rocks, by the evidently terrestrial origin of the surfaces on 

 which they lie, and by the intercalation among them of old soils, 

 forests, mud-streams, river-gravels, lake-deposits, and masses of 

 unstratified tuffs and ashes. From the analogy of existing volcanic 

 districts, we can scarcely doubt that these great accumulations 

 of igneous products, which must originally have covered many 

 thousands of square miles, and which still often exhibit a thickness 

 of 2000 feet, were ejected from great volcanic mountains ; and 

 a careful study of the district fully confirms this conclusion, 

 enabling us, indeed, to determine the sites of these old volcanoes, to 

 estimate their dimensions, to investigate their internal structure, 

 and to trace the history of their formation. 



The Tertiary Volcanoes. — The petrology of the Western Isles has 

 been made the subject of careful study by Professor Zirkel, of 

 Leipzig, to whose investigations we are very deeply indebted. The 

 Tertiary igneous rocks may be classified, according to their ultimate 

 chemical composition, into two series, known as the acid and basic 

 igneous rocks. In each of these series the proportions of the 

 several ingredients in its various members are almost identical ; but 

 in structure the rocks of either series vary from the coarsest 

 crystalline aggregates to the most perfect glass. The acid series 

 consists of granite, felsite, felstone, and pitchstone ; the basic of 

 gabbro, dolerite, basalt, and tachylite ; the members of either series 

 exhibit innumerable varieties, and pass into one another by the most 

 insensible gradations. The igneous rocks of both classes form : — 

 lava-streams, often of great thickness and extent, and exhibiting 

 many interesting peculiarities of the amygdaloidal and columnar 

 structures ; eruptive masses, varying in size from great mountain- 

 groups to the smallest dykes and veins ; volcanic agglomerates, com- 

 posed of the scoriae and ashes ejected from volcanic vents; and 

 volcanic breccias, made up in great part of the fragments of the 

 various Palaeozoic and Secondary rocks through which the volcanoes 

 have burst. Among the volcanic agglomerates are found beautiful 

 examples of the more stable of the species of minerals characteristic 

 of the neighbourhood of volcanic vents. 



The relations of these several igneous products to one another are 

 beautifully exemplified in the Island of Mull. We here find proof 

 that the volcanic activity of the Tertiary period commenced with 

 the eruption of felspathic lavas and associated fragmentary materials. 



