474 Geikie — Volcanic Rocks of Great Britain. 



rating the Miocene basalts from the New Eed Sandstone trap-rocks, 

 which seem to come next to them in point of recentness, was im- 

 mensely vaster than that which has elapsed between the Miocene 

 basalts and the present time. There is thus no improbability in the 

 eventual outbreak once more of the subterranean forces. Nay, fur- 

 ther, were a renewed series of volcanic eruptions to take place now, 

 they might in 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 interesting and, at the same time, most full of difficulty, 

 is that of the trap-dykes. To my own mind there are few parts of 

 the geology of the country so hard to understand as the extravasa- 

 tion 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, and extending for leagues, 

 to a distance of fully 200 miles from the nearest point of the basaltic 

 plateaux. Did they reach the surface originally ? If so, were they 

 connected with outflows of dolerite, now wholly removed by denuda- 

 tion ? I confess that this supposition has often presented itself to 

 me as carrying with it much probability. It seems to me unlikely 

 that so many thousands of dykes should have risen so high as the 

 present surface, retaining there (as shown by deep mines) much the 

 same proportions as they show many fathoms down, and yet that 

 none of them should have reached the surface which existed at the 

 time of eruption. I regard it as much more probable that some of 

 them, at least, rose to daylight, and flowed out as coulees, even over 

 parts of the south of Scotland and north of England, where all trace 

 of such surface masses has long been removed. Some of the surface- 

 masses of dolerite in these districts may indeed be of Tertiary age ; 

 yet the proofs which the great Miocene basaltic plateaux present of 

 enormous denudation are so striking as to make the total disappear- 

 ance of even wide and deep lava-currents quite conceivable. 



But a much more serious difficulty remains. These dykes, as a 

 rule, do not come up along lines of fault, yet they preserve wonder- 

 fully straight courses, even across fractured and irregular strata. 

 Each dyke retains, as a rule, a tolerably uniform breadth, and its 

 sides are sharply defined, as if a clean, straight fissure had been 

 widened and filled up with solid rock. More than this, they are 



^ Boue felt this difficulty, but he conceived that the fissures had been filled from 

 above by masses of basalt, erupted at diff'erent points, and spreading over the country, 

 though now removed by denudation. He says : — " Nous croyons infiniment probable 

 que ces filons ont tous ete formes de meme [i.e., remplis par des courans de lave dans 

 leur marche], malgre les grandes destructions qu' entraine cette supposition, et que 

 rarement il y en a eu quelques-uns qui ont ete remplis lateralment ou de dilferentes 

 manieres bizarres." — Geol. d' Hcosse, p. 272. 



