84 Lower Silurian Rocks, 



with regard to these ancient boulder beds is held by 

 Professor Greikie and Mr. James Geikie, who mapped 

 the country. 1 



The Lower Silurian rocks of the south, pass under- 

 neath the Old Eed Sandstone and Carboniferous rocks 

 of the midland parts of Scotland, and rise again on the 

 north in the Grampian Mountains. A great fault, 



1 I shall by-and-by have to notice the recurrence of glacial 

 episodes at various epochs in geological history, a subject with 

 which ever since 1855 I have had a good deal to do. (On Permian 

 Breccias, &c. ' Journal of the Geol. Soc.' vol. ii. p. 185). It is diffi- 

 cult to make out the ground on which all the old, and many of the 

 middle aged geologists, have cast aside the various evidences that 

 have been adduced in support of the recurrence of glacial epochs 

 or episodes, especially as I remember no argument that has been 

 brought forward, excepting that in old times, the radiation of in- 

 ternal heat through the crust of a cooling globe, produced warm and 

 uniform climates all over the surface, and that the further you go 

 back in time the hotter they were. The Lias was accumulated in 

 warm seas, and if so those of the Carboniferous times were warmer, 

 and those of Silurian times warmer still, and I have heard a dis- 

 tinguished geologist declare in a public lecture, that the tropical 

 vegetation of the Coal-measures, was due to the heat that radiated 

 outwards from the earth's crust, aided by that produced by the flaring 

 volcanoes of that epoch ! Undoubtedly there must have been a geo- 

 logically prehistoric time, when internal heat may have acted on the 

 surface, and perhaps the sun may have been hotter than now, and 

 that also had its effect. I, however, can see no signs of these internal 

 and external interferences since the times in which the authentic 

 records of geological history have been preserved, and these extend 

 backward earlier than the Lower Silurian epoch. I recollect the 

 time when what passed for strong arguments were urged to prove 

 that the former great extension of the Alpine glaciers advocated by 

 Agassiz, and the existence of glaciers in the Highlands, Cumberland, 

 and Wales, proved by him and Buckland, were mere myths. Now, 

 however, there is such a persistent run upon the subject, that more 

 memoirs have been, and still are being, written about it, than per- 

 haps on any other geological question. Coincident with this a 

 beginning of the acceptance of the theory of the recurrence of 

 glacial episodes, is slowly making its way both in England and the 

 Continent. 



