338 G. W. Bulman — Glacial Geology. 



lying one another throughout considerable areas, and these clays 

 are described as being distinctly separate and distinguishable the 

 one from the other." And further : — " Penck, Bohn, and Briickner 

 find evidence of two interglacial epochs, and maintain that there 

 have been three distinct and separate epochs of glaciation in the 

 Alps." » 



And according to Dr. Croll's views, there must have been a greater 

 number. For his calculations indicate that the glacial epoch lasted 

 160,000 years, and dividing this into periods of 10,000 — the time 

 allowed for a warm interval — we get eight such mild interglacial 

 periods. 



Prof. Geikie himself divides the ice age into periods as follows : ^ — 

 1) Pre-glacial mild period First bed. 



2) First glacial period 



3) First interglacial mild period 



4) Second glacial period 



5) Second interglacial mild period 



6) Third glacial period 



7) Third and last interglacial mild \ Hessle gravel, middle sands, etc., 



Cromer Clay and contorted beds, 

 f Sand and rolled gravel above 

 \ Cromer clay. 



Great chalky boulder-clay, etc. 

 f Beds between great chalky boulder- 

 \ clay and purple clay. 

 I Purple clay, lower boulder-clay of 

 ( Lancashire. 



of Lancashire. 

 Hessle clay, upper boulder-clay 

 of Lancashire. 



period 

 8) Last glacial period 



"With regard to the first point in the evidence, two suggestions 

 occur : 



(1) Beds of sand intercalated in the till may be in part the result 

 of the action of the ice itself. In fact, if we do not so regard them, 

 the composition of the till and boulder clay is a diflSculty on the 

 usual hypothesis of its origin. The products of glaciation are 

 supposed to be these clays, and morainic debris : that is to say, very 

 finely ground rock matter, forming the clay, with large and small 

 stones making up the rest of the till and the morainic matter. But 

 in the process of grinding down there must have been an inter- 

 mediate product, viz. coarse and fine sand ; and if these are not 

 represented by the intercalated sand beds, where are they ? And 

 even if all the rock matter excepting the boulders and pebbles had 

 been ground down to the fineness of the sediment forming the clay, 

 it is difficult to understand how a purely siliceous sand could ever 

 become plastic, and form a clay. For although the glacial clay is 

 at times sandy, its general composition does not permit the suppo- 

 sition that it contains all the sand produced by the grinding down 

 of the rocks by the ice. In fact, it seems almost certain that sand 

 must necessarily be one of the results of ice action ; and that it 

 should occur interstratified with the till is as natural as that in a 

 river deposit sand should be intercalated with gravel. 



And in his " Ice Age in North America," Dr. Wright speaks of 

 "the sands and gravels of the terminal moraine" of the Muir 

 glacier (p. 47). 



1 See Brit. Assoc. Eeports, 1889, pp. 556-558. 



2 Great Ice Age, p. 393. 



