306 James, Croll — On the Glacial Epoch. 



Fig. 3. — Aviculopecten elliptieus, Phillips. Natural size. Shale of the L. Carbon- 

 iferous Limestone series. Teiglam Burn, Lesmahagow. 

 Figs. 4, 5, and 6. — Posidonomya corrugata, Etheridge. Shale above the Calderwood 

 Cement Stone, L. Carboniferous Limestone Group, Kirktonholm Cement 

 "Works, E. Kilbride. 

 For the specimen from which Fig. 1 is taken I am much indebted to Mr. James 

 Bennie, who obtained it from the locality above mentioned. The original of Fig. 3 was 

 kindly lent me by Mr. C. "W. Peach; and those from which Figs. 4, 5, and 6 were 

 drawn, are from the collection of Mr. A. Paton, of E. Kilbride, who obtained them at 

 the locality mentioned, and to whom I am much indebted for an opportunity of 

 examining numerous other fossils from that district. 



III. — On the Physical Cause of the Submergence and Emergence 



OF the Land during the Glacial Epoch. 



By Jambs Croll, of the Geological Survey of Scotland, 



AN ordinary glacier descends in virtue of the slope of its bed, 

 and, as a general rule, it is on this accounb thin at its com- 

 mencement, and thickens as it descends into the lower valleys, 

 where the slope is less and the resistance to motion greater. But 

 in the case of continental ice, matters are entirely different. The 

 slope of the ground exercises little or no influence on the motion of 

 the ice. In a continent of one or two thousand miles across, the 

 general slope of the ground may be left out of account ; for any 

 slight elevation which the centre of such a continent may have will 

 not compensate for the resistance offered to the flow of the ice by 

 mountain ridges, hills, and other irregularities of its surface. The 

 ice can move off such a surface only in consequence of pressure 

 acting from the interior. In order to produce such a pressure, 

 there must be a piling up of the ice in the interior ; or, in other 

 words, the ice-sheet must thicken from the edge inwards to the 

 centre. We are necessarily led to the same conclusion, though we 

 should not admit that the ice moves in consequence of pressure from 

 behind, but should hold, on the contrary, that each particle of ice 

 moves by gravity in virtue of its own weight ; for in order to have 

 such a motion, there must be a slope, and as the slope is not on the 

 ground, it must be on the ice itself : consequently, we must conclude 

 that the upper surface of the ice slopes upwards from the edge to 

 the interior. What, then, is the least slope at which the ice will 

 descend ? Mr. Hopkins found that ice barely moves on a slope of 

 one degree. We have therefore some data for arriving at least at a 

 rough estimate of the probable thickness of an ice-sheet covering a 

 continent, such, for example, as Greenland or the Antarctic Continent. 

 The Antarctic Continent is generally believed to extend, on an 

 average, from the South Pole down to about, at least, latitude 70°. 

 In round numbers, we may take the diameter of this continent at 

 2800 miles. The distance from the edge of this ice-cap to its 

 centre, the South Pole, will, therefore, be 1400 miles. The whole 

 of this Continent, like Greenland, is undoubtedly covered with 

 one continuous sheet of ice gradually thickening inwards from its 

 edge to its centre. A slope of one degree continued for 1400 miles 

 will give twenty-four miles as the thickness of the ice at the Pole. 

 But suppose the slope of the upper surface of the cap to be only one- 

 half this amount, viz. a half degree, — and we have no evidence that a 



