126 SCIENCE IN SHORT CHAPTERS. 



they are found imbedded in the till, i.e., the oblong frag- 

 ments lying with their longer axes and their best marked 

 striae in the direction of the motion of the glacier. The 

 "striated pavements" of the till are thus easily explained; 

 they are the surface upon which the ice advanced when its 

 deposits had reached the critical or neutral height. Such 

 a pavement would continually extend outwards. 



The only sorting of the material likely to occur under 

 these conditions would be that due to the earlier deposi- 

 tion and entanglement of the larger fragments, thus pro- 

 ducing a more stony deposit nearer inland, just as Mr. 

 Geikie describes the actual deposits of till where, "gener- 

 ally speaking, the stones are most numerous in the till of 

 hilly districts; while at the lower levels of the country the 

 clayey character of the mass is upon the whole more pro- 

 nounced." These " hilly districts," upon the supposition 

 of greater submergence, would be the near shore regions, 

 and the lower levels the deeper sea where the glacier floated 

 freely. 



The following is Mr. Geikie's description of the distribu- 

 tion of the till (page 13) : " It is in the lower-lying districts 

 of the country where till appears in greatest force. Wide 

 areas of the central counties are covered up with it con- 

 tinuously, to a depth varying from two or three feet up to 

 one hundred feet and more. But as we follow it towards 

 the mountain regions it becomes thinner and more inter- 

 rupted the naked rock ever and anon peering through, 

 until at last we find only a few shreds and patches lying 

 here and there in sheltered hollows of the hills. Through- 

 out the Northern Highlands it occurs but rarely, and only 

 in little isolated patches. It is not until we get way from 

 the steep rocky declivities and narrow glens and gorges, 

 and enter upon the broader valleys that open put from the 

 base of the highland mountains to the low-lying districts 

 beyond, that we meet with any considerable deposits of 

 stony clay. The higher districts of the Southern Uplands 

 are almost equally free from any covering of till.' 



This description is precisely the same as I must have 

 written, had I so far continued my imaginary sketch of the 

 results of ancient glaciation as to picture what must re- 



