J. Croll — Evidence of former Glacial Periods. 67 



all. ISTot only are the geological records of ancient glacial con- 

 ditions imperfect, but this imperfection follows as a natural 

 consequence from the principles of geology itself. There are 

 not merely so many blanks or gaps in the records, but a reason 

 exists in the very nature of geological evidence why such 

 breaks in the record might naturally be expected to occur. 



The evidence of Glaciation is to be found chiefly on land- 

 surfaces. — It is on a land-surface that the principal traces of 

 the action of ice during a glacial epoch are left, for it is there 

 that the stones are chiefly striated, the rocks ground down, and 

 the bowlder clay formed. But where are all our ancient land- 

 surfaces ? They are not to be found. The total thickness of 

 the stratified rocks of Great Britain is, according to Professor 

 Ramsay, nearly fourteen miles. But from the top to the bot- 

 tom of this enormous pile of deposits there is hardly a single 

 land-surface to be detected. Patches of real old land-surfaces 

 of a local character may indeed be found, as, for example, the 

 dirt-beds of Portland ; but, with the exception of coal-seams, 

 every general formation has been accumulated under water, 

 and none but the under-clays ever existed as a land-surface. 

 And it is here, in a general formation, that the geologist has to 

 collect all his information regarding the existence of former 

 glacial epochs. The entire stratified rocks of the globe, with 

 the exception of the coal-beds and under-clays (in neither of 

 which would one expect to find traces of ice-action), consist 

 almost wholly of a series of old sea-bottoms, with here and 

 there an occasional fresh-water deposit. Bearing this in mind, 

 what is the sort of evidence which we can now hope to find 

 in these old sea-bottoms of the existence of former ice- 

 periods ? 



All geologists of course admit that the stratified rocks are 

 not old land-surfaces, but a series of old sea-bottoms formed 

 out of the accumulated material derived from the degradation 

 of primeval land-surfaces. And it is true that all land-surfaces 

 once existed as sea-bottoms ; but the stratified rocks consist of 

 a series of old 'sea-bottoms which never were land-surfaces. 

 Many of them no doubt have been repeatedly above the sea- 

 level, and may once have possessed land-surfaces ; but these, 

 with the exception of the under-clays of the various coal 

 measures, the dirt-beds of Portland, and one or two more 

 patches, have all been denuded away. The important bearing 

 which this consideration has on the nature of the evidence 

 which we can now expect to find of the existence of former 

 glacial epochs has certainly been very much overlooked. 



If we examine the matter fully we shall be led to conclude 

 that the transformation of a land-surface into a sea-bottom 

 will probably completely obliterate every trace of glaciation 



