xiv The Record of the Rocks 271 



and mollusca. The cause of changes of climate, sufficient 

 to produce such effects, has been the subject of much specu- 

 lation. The late Dr. Croll, whose theory is now most widely 

 received, pointed out that the changes in the eccentricity 

 of the Earth's orbit ( 109) combined with the precession of 

 the equinoxes ( 115), must have produced a severe climate 

 in the northern hemisphere at the period when aphelion 

 occurred in the northern winter, and the eccentricity was at 

 a maximum. If Croll's theory is true, cold periods must 

 have occurred in all geological epochs. Erratic blocks and 

 glaciated stones found in many different formations seem to 

 confirm it, but no sign has been found of such extensive 

 ice-action as characterised the Pleistocene. This may be 

 accounted for by the probable absence in those remote 

 periods of continental areas sufficiently extensive to support 

 a great ice-sheet. Some geologists account for the changes 

 of level during this period by supposing that the great ice- 

 sheet depressed the elastic strata by its weight, producing 

 extensive subsidence, followed by upheaval when the ice- 

 cap melted. Others explain raised beaches ( 284) on the 

 assumption that the land remained rigid and the mass of 

 ice raised the level of the ocean by attraction ( 2 52). In the 

 river and cave accumulations of the Pleistocene age the first 

 undoubted signs of the human race appear in the form of 

 coarse chipped stone implements and rough etchings on bone 

 of extinct or no longer indigenous animals. 



353. Evolution of Continents. Rocks of Archaean and 

 Palaeozoic age cover a greater area on the Earth than those of 

 Mesozoic age, which are in turn more extensive in their dis- 

 tribution than those of the Tertiary system. This shows that 

 more of the elevated half of the globe was covered by the sea, 

 in which sediment aocumulated, in Palaeozoic than in Meso- 

 zoic, and in Mesozoic than in Tertiary times. It is pointed 

 out by Professor J. Geikie that the elevated and depressed 

 halves of the World have been growing more and more 

 distinct throughout geological ages, and as the Abysmal Area 

 has grown deeper and the World Ridges higher the superficial 

 extent of the hydrosphere has been steadily diminishing, 

 although its volume remains the same. 2 This change must 



