53 REPORT — 18G7. 



and west trend, and cutting througli all the other roclcs. Hero, then, in this little 

 tract, about the size of a small English county, there are the chronicles of a long 

 series of volcanic eruptions, bepinning in the" middle of the Old Red Sandstone, 

 and coming down to a time relatively so near oiu- own as that of the Miocene 

 rocks. Nor is this by any means an exceptional district. Illustrations of a similar 

 persistence of volcanic action may be gathered in many other tracts of equally 

 limited extent. 



Another fact which a general survey of the character of our volcanic rocks 

 soon brings before us, is that, as a whole, those of earlier date diifer distinctively 

 in composition from those of more recent origin. From the first traces of volcanic 

 activity in this countrv up to about the close of the Old Red Sandstone or begin- 

 ning of the Carboniferous series, the interbedded (i. e. contemporaneous) igneous 

 rocks consist for the most part of highly felspathic masses, to which the names 

 of clinkstone, clavstone, compact felspar, porphyry, hornstone, felstone, &c. have 

 been given. On "tlie other liand, from the upper part of the Old Red 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 aud dolerit(!s (or gi-een- 

 stones, as the latter ha-^e been usually termed in Scotland). Were these rocks 

 subjected to further aud more detailed chemical examination, additional know- 

 ledge might possibly be acquired respecting the history of thechanges which have 

 taken place witliin "the crust of the earth." As geologists, it is important 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 iu their 

 mode of occurrence. Certain minor differences are, indeed, readily observable, 

 such as the greater thickness of the beds among the older rocks, and the more 

 frequent occurrence of columnar structure among the newer. Perliaps these and 

 other distinctions may eventually give us a general type for each class. Never- 

 theless, in its broader" features there would seem to have been a striking -unifor- 

 mity iu volcanic action from the earliest times down to our owu day. 



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

 no proofs that volcanic action lias been slowly diminishing in intensity during past 

 geological time. The amount of volcanic material preserved in our Old Red Sand- 

 stone group probably exceeds that of our Silurian system, even after all due allow- 

 ance for the greater denudation of the older .series. The number of distinct 

 volcanic centres traceable among the Carboniferous rocks in like manner surpasses 

 that of the earlier formations. I3ut 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 -3000 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 separating the Miocene basalts from the New Red Sandstone trap-rocks, 

 which seem to come next to them iu point of recentness, was immensely vaster 

 than that which has elapsed between the Miocene basalts aud the present time. 

 There is thus no improbability in the eventual outbreak once more of the subter- 

 ranean forces. Nay, further, were a renewed series of volcanic eruptions to take 

 place now, they might iu the far distant future be thrown together with those of 

 Miocene date, as proofs of one long period of interrupted volcanic activity, just as 

 we now group the igneous rocks of the Lower Silurian, or of any other geological 

 formation : so near to us, in a geological sense, are those latest and grandest of our 

 volcanic phenomena. 



Among the different forms assumed by our igneous rocks, one of the most in- 

 teresting, and, at the same time, most full of difficulty, is that of the trap-dykes. 

 To my own mind tliere are few parts of the geology of the couutiy so hard to 

 imderstand as the extravasation of the thousands of dykes by which the north- 

 western portion of this island is so completely traversed. For the reasons already 

 assigned, I would refer the leading system of these dykes to the same geological 

 age as the Tertiary volcanic rocks of the north-west. Yet we find them rising to 

 the surface, aud extending for leagues, to a distance of fully 200 miles from the 

 nearest point of the basaltic plateaux. Did they reach the surfixce originally ? If 



