1855.] RAMSAY PERMIAN BRECCIA. 203 



In connection with this supposed universality of tropical climate, 

 it has been objected that the nature of the Permian fauna and flora 

 affords an argument against the possibility of glaciers existing in 

 Permian times in this area, more especially as the Permian flora suc- 

 ceeds and nearly resembles the flora of the Coal-measures, supposed 

 by many to have a tropical character. To this it may be replied — 1 st. 

 That there is nothing in the Permian marine fauna essentially tropical, 

 and of the habits of the one solitary Labyrinthodon we are altogether 

 ignorant. 2nd, It was the opinion of Dr. Mantell, and has been 

 confirmed by Dr. J. D. Hooker, that the Carboniferous flora indi- 

 cates, not a tropical, but a moist, equable, and temperate climate*, 

 possibly such as that of New Zealand ; in which country, it will be 

 remembered, there are glaciers at the present day in the southern 

 island f. If indeed, after the early stage of growth, the beds of 

 carbonaceous matter that formed coal accumulated in a manner at 



the temperature of rocks would rise to the melting-point of lava at 113,100 feet 

 beneath the surface, assuming for these latitudes a constant temperature of 50° at 

 the depth of 60 feet. It does not, however, therefore follow, that they should melt 

 at that depth, for this might be interfered with by pressure ; but it may be assumed 

 that under such circumstances the rocks might be considerably altered, if subjected 

 to this high temperature for a great length of time. I attribute, for instance, in 

 some cases, the metamorphism of shales or slates into gneiss to their approaching 

 the sphere of such influences. Now (on the authority of Mr. W. Hopkins, Quart. 

 Journ. Geol. Soc. vol. viii. p. 59), " the present effect of the internal heat is about 

 one-twentieth of a degree" on the mean superficial temperature ; but to affect the 

 external climates of the globe 1° Fahr. (namely twenty times the present amount) 

 " the descending rate of increase must have been twenty times as great as at pre- 

 sent, about 20° Fahr, for every 60 feet ; and, if the superficial temperature were 

 thus raised about 10° Fahr., the temperature at the depth of 60 feet would, ac- 

 cording to the same law, exceed 200° Fahr., and all but surface-springs would be 

 springs of boiling-water. This physical state of our planet would scarcely, per- 

 haps, be deemed consistent with the conditions of animal life at the more recent 

 geological epochs." To this conclusion many geologists are steadily arriving ; 

 and for many years I have held that internal heat, at least since the formation of 

 the oldest fossiliferous rocks, has not materially modified the climates of the world. 



On the foregoing data an argument in favour of this view may be drawn from 

 the rocks themselves. Let us suppose that the external climate was affected by 

 internal heat only 1° Fahr., involving an increment of 20° Fahr. for every 60 feet 

 of descent, then, instead of reaching the equivalent temperature of the surface- 

 melting-point of lava at 113,100 feet beneath the surface, we should reach it at a 

 depth of about 5700 feet; and, were the average surface-temperature incj^eased 

 10°, we should reach it at a depth of about 580 feet. The thicknesses of many 

 British formations have been determined by the Geological Survey. Thus in 

 North Wales the Barmouth and Harlech grits are about 7000 feet thick without 

 our reaching their base, and 25,000 feet of Lower Silurian strata are conformably 

 superimposed on these, giving a total of 32,000 feet. But, as a rule, the rocks 

 are not highly altered. The slates are cleaved and hardened ; but it is only in a 

 few places, where granite or other allied rocks have been intruded into them, that 

 they become so changed as to deserve the title of metamorphic. 1 have elsewhere 

 shown (Quart. Journ. Geol. Soc. vol. ix. p. 170), that the Welsh igneous bosses 

 that effect this alteration were simply the nuclei or centres of the Lower Silurian 

 volcanos ; and in areas removed from these the rocks remain unmetamorphosed ; 

 which ought not to have been the case, had the rocks in general attained and 

 long maintained the temperature of melting lava at a depth of only 5700 feet. 



* Memoirs of the Geological Survey, vol. ix. part 2. p. 399. 



t I derived this information from the late Dr. Mantell, who informed me that 

 he had received descriptions of them from his son. 



