﻿206 Mr. R. Peacock on Geological Time, and the probable 



by its mechanical energy, together with the weight of the glacier, 

 be sufficient to account for the motion. But the mechanical 

 energy of heat is not required to push the glacier forward; 

 gravitation alone, as we have just seen, will suffice. Besides, 

 heat entering ice could not produce a mechanical pressure that 

 would move the glacier ; for heat produces contraction of volume, 

 not expansion. True, heat no doubt destroys the crystalline 

 structure of the ice-molecule by tearing the constituent particles 

 separate ; but nevertheless the volume of the mass is diminished 

 by this process, for ice in losing its crystalline structure, or, in 

 other words, in passing from ice to water, decreases in volume. 



XXVII. On Mr. J. GrolPs paper " On Geological Time, and the 

 probable Date of the Glacial and the Upper Miocene Period"*. 

 By R. A. Peacock, C.E.f 



THE writer hereof believes that Mr. Croll's paper is of great 

 value ; and if a few remarks are ventured upon below on 

 small points of detail, they no more detract from the general 

 value of the paper than a few " strise " or scratches would detract 

 from the value of a good painting. 



" The only evidence which we can now reasonably expect to 

 find in the stratified rocks of the existence of land-ice of former 

 epochs, is the presence of erratic blocks which may have been 

 transported by icebergs and dropped into the sea. But unless 

 the glaciers of that epoch reached the sea or the sea was frozen, 

 we could not possibly have even this evidence. Traces in the 

 stratified rocks of the effects of land-ice of former epochs must, 

 from the very nature of things, be rare indeed" (p. 364). 



On the contrary, might we not have striation on the stratified 

 rocks pretty often in this way ? When we remember the fre- 

 quent oscillations of land by sinkings and risings in every part 

 of the globe since the commencement of the glacial period (assu- 

 ming that to have commenced 240,000 years ago and to have 

 lasted 160,000 years), the following may often have happened. 

 Suppose (as must have been the case) many glaciers to have 

 been making each its way down its own valley in the usual 

 manner, bearing its lines of moraines, as in Switzerland at pre- 

 sent. The glaciers would then striate the stratified rocks of evei'y 

 valley. Such striations would continually go on increasing as 

 long as the glaciers existed. And considering the vast tract of 

 the earth which must have been thus operated on during 160,000 



* Philosophical Magazine, November 1868. 

 f Communicated by the Author. 



