Geikie — Volcanic Rocks of Great Britain. 473 



of these rocks there is an excess of silica (55 to 80 per cent.), which 

 is sometimes fomid separated out into distinct granules. On the 

 other hand, from the upper part of the Old Ked Sandstone, or the 

 lower members of the Carboniferous series, up to the end of the 

 long history, the erupted masses are chiefly augitic, as basalts and 

 dolerites (or greenstones as the latter have been usually termed in 

 Scotland). In these rocks free silica is not a normal constituent, 

 while the alkalies, alkaline earths, and metallic oxides form on an 

 average about half of the whole mass. In the former class the acid 

 element predominates, in the latter the bases are specially con- 

 spicuous. According to Durocher the earlier series arose from an 

 upper acid magma, while those of later age came up from an under- 

 lying basic magma. Were these rocks subjected to further and 

 more detailed chemical examination, additional knowledge might 

 possibly be acquired respecting the history of the changes which 

 have taken place within the crust of the earth. 



As geologists, however, it is important for us to note that, though 

 two classes of volcanic rocks can thus be determined by analysis 

 of their composition, no broad essential distinctions appear to be 

 traceable in their mode of occurrence. The earlier volcanos, which 

 threw out siliceous lavas and ashes, seem to have acted very much 

 in the same way as those of later date, which gave out the heavier 

 pyroxenic lavas. Certain minor differences are indeed readily ob- 

 servable. Thus the older lavas occur as a rule in much thicker 

 beds than the later ones, which, indeed, are distinguished by that 

 markedly bedded character which results from the number and thin- 

 ness of their successive flows. As a concomitant of this arrange- 

 ment also, columnar structure is much more frequent among the 

 pyroxenic than among the silicious rocks. Perhaps, if these and 

 other distinctions were collected and compared, each class of rocks 

 might be found to possess certain characteristic peculiarities of its 

 own, sufficient when taken together to give us a type for general 

 reference. Nevertheless, in its broader features, there would seem 

 to have been a striking uniformity in volcanic action from the 

 earliest times down to our own day. 



This leads me to remark that a study of the igneous rocks of 

 Britain furnishes no proofs that volcanic action has been slowly 

 diminishing in intensity during past geological time. The amount 

 of volcanic material preserved in our Old Eed Sandstone group 

 probably exceeds that of our Silurian system, even after all due 

 allowance for the greater denudation of the older formation. The 

 number of distinct volcanic centres traceable among the Carbonifer- 

 ous rocks in like manner surjiasses that of the older formations. 

 But by much the most extensive mass of volcanic material in these 

 Islands belongs to the latest epoch of eruption — that of the Miocene 

 period. In one mountain alone, Ben More in Mull, these youngest 

 lavas rise over each other, tier above tier, to a height of more than 

 3,000 feet ; yet their base is concealed under the sea, and their top 

 has been removed by denudation. We have here, therefore, no 

 proof of a slow diminution of volcanic activity. The period sepa- 



