CHAP. XIII.] PLEISTOCENE EPOCH. 279 



was subsequently laid down upon it ; and further, that the 

 conditions which permitted the clay to accumulate were 

 different from those which allowed the rock to be scratched. 



Finally, those who hold the land-ice theory are com- 

 pelled to admit not one Glacial period only, nor even two, 

 but an indefinite number of such periods, separated by an 

 equal number of non- glacial and comparatively warm 

 periods, solely because beds of stratified sand and gravel 

 containing marine shells are frequently found between 

 sheets of Boulder-clay. Every such bed is for them a 

 record of an " Inter glacial period," and each sheet of 

 Boulder-clay is the record of a distinct Glacial period sepa- 

 rated from that of the clay below by an enormous interval 

 of time ! To anyone who is familiar with the Glacial 

 series of England, where such intercalations of sand, 

 gravel, and loam are frequent, such a view must seem a 

 very forced and unnatural method of explanation. 



Professor J. Geikie's presentation of the ice-sheet theory 

 is,^ however, by no means the only way in which this 

 theory ca.n be applied, and it is quite conceivable that a 

 modification of it is capable of explaining the Glacial phe- 

 nomena of Scotland. We may grant that the moulding 

 and grooving of the rock surface has been accomplished by 

 an ice-sheet composed of confluent Scottish glaciers, and 

 indeed few could read Dr. Archibald Geikie's admirable 

 descriptions of Scottish scenery 1 without admitting the 

 former presence of such an agent ; but we must urge that 

 some change took place before this surface could have been 

 so largely covered with Boulder-clay. A possible supposi- 

 tion is that the ice-sheet remained, but that a general subsi- 

 dence of the whole country occurred ; the detritus which 

 was being scoured off the face of the land would then come 

 to rest where the pressure of the ice-sheet was relieved by 



1 " The Scenery of Scotland/' second edition, 1887. 



