J, W. Judd — Secondary Rocks of Scotland. 69 



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

 unstratified tuifs 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 volcanos, to estimate 

 their dimensions, to investigate their internal structure, and to trace 

 the history of their formation. 



The Tertiary Volcanos. — 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 crystal- 

 line 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 in- 

 sensible 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, composed of the 

 scoriee and ashes ejected from volcanic vents ; and volcanic breccias 

 made up in great part of the fragments of the various Paleozoic and 

 Secondary rocks through which the volcanos 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. 

 These were accompanied by the intrusion among the surrounding 

 strata, which they greatly metamorphosed, of great masses of fluid 

 rock of acid composition, and the extrusion among the other igneous 

 products of similar liquefied materials, which consolidated into 

 felsite and granite. The grea^t volcanic mountains thus formed 

 appear to have remained in a state of comparative quiescence for a 

 vast period, during which they were subjected to great denudation, 

 and then through their midst were forced great masses of fluid 

 basaltic rocks, which continued to flow at intervals during enormous 

 periods, and gave rise to streams of basalt which accumulated to the 

 thickness of many hundreds and even thousands of feet. The great 

 intrusive bodies of this same rock consolidated into mountain masses 

 of gabbro and dolerite. While the earlier felspathic lavas appear to 



