346 James Croll — On the Glacial Epoch. 



siding rocky matter, it might be found in the fact that the traces of 

 extreme crushing, such as corrugation, slaty cleavage, etc., do not 

 occur in volcanic regions — which rarely show signs of much dis- 

 turbance, but only dykes, i.e. narrow fissures filled with lava-rock, 

 cutting through undisturbed, or but slightly disturbed strata, making 

 it probable (as I said in my first notice of Mr. Mallet's theory) 

 that if any heat is, or ever was, generated by the compression of 

 rocks, its effects should be looked for rather in the metamorphosed 

 and plicated rocks of mountain chains than in the lines of volcanic 

 eruption. 



I must apologize to the readers of the Geological Magazine for 

 this continued controversy with Mr. Mallet, on the double ' ground 

 of, 1st, that gentleman's repeated and persevering misrepresentation 

 of my views on volcanic action, and 2ndly, the inherent weakness of 

 the theory he has advanced with so much pretension as " a precise 

 and true theory, in substitution for the current and erroneous notions 

 as to the nature of those forces, from the play of which, within our 

 globe, those grand phenomena of nature (volcanos) are produced." 

 —Phil. Trans, p. 147. 



III.-^-On the Physical Cause of the Submergence and Emergence 



OF the Land during the Glacial Epoch. 



By James Croll, of the Geological Survey of Scotland. 



[Continued from page 314.) 



THE greatest extent of the displacement of the earth's centre of 

 gravity, and consequently the greatest rise of the ocean result- 

 ing from that displacement, would of course occur at the time of 

 maximum glaciation, when the ice would be all on one hemisphere. 

 But owing to the following circumstance, a still greater rise than 

 that resulting from the displacement of the earth's centre of gravity 

 alone might take place at some considerable time, either before or 

 after the period of maximum glaciation. 



It is not at all probable that the ice would melt on the warm 

 hemisphere at exactly the same rate as it would form on the cold 

 hemisphere. It is probable that the ice would melt more rapidly on 

 the warm hemisphere than it would form on the cold. Suppose 

 that during the Glacial ej)och, at a time when the cold was gradually 

 increasing on the northern and the warmth on the southern hemi- 

 sphere, the ice should melt more rapidly off the Antarctic Continent 

 than it was being formed on the Arctic and Sub-arctic regions; 

 suppose also that, by the time a quantity of ice, equal to one-half 

 what exists at present on the Antarctic Continent, had accumulated 

 on the northex-n hemisphere, the whole of the Antarctic ice had been 

 melted away, the sea would then be fuller than at present by the 

 amount of water resulting from the one mile of melted ice. The 

 height to which this would raise the general level of the sea would 

 be as follows : — 



The Antarctic ice-cap is equal in area to W-t-b- of that covered by 

 the ocean. The density of ice to that of water being taken at -92 

 to 1, it follows that 25 feet 6 inches of ice melted off the cap would 



