364 Mr. J. Croll on Geological Time, and the probable 



land- surfaces. And it is true that all land-surfaces once existed 

 as sea-bottoins ; 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 coal-measures, the dirt-beds of Portland, 

 and one or two more patches, have all been annihilated. The 

 important bearing that 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 over- 

 looked. 



When we examine the matter fully, we find that the transfor- 

 mation of a land-surface into a sea-bottom will probably com- 

 pletely obliterate every trace of glaciation that may have existed 

 on that land-surface. For example, we cannot expectto find the 

 polished and striated stones belonging to a former land-glacia- 

 tion ; for stones are not carried down by our rivers and deposited 

 in the sea. They are first disintegrated by subaerial agency into 

 sand or clay, as the case may be, and then carried down by our 

 rivers and deposited as such on the sea-bottom. But supposing 

 striated stones were carried down by our rivers out of the boulder- 

 clay, they could not retain their ice-markings, for they would 

 soon become waterworn in their passage seawards. 



Neither can we expect to find boulder-clay in the stratified 

 rocks, for boulder-clay is not carried down as such and deposited 

 in the sea. The boulder-clay is washed off the land and car- 

 ried dowm as soft mud, clay, sand,, and gravel. Patches of 

 boulder-clay may have been now and again forced into the sea 

 by means of the ice and become covered up ; but such cases are 

 wholly exceptional, and the absence of examples of this sort in 

 any formation cannot fairly be adduced as a proof that that for- 

 mation does not belong to a glacial period. 



The only evidence which we can now reasonably expect to find 

 in the stratified rocks of the existence of land-ice of former epochs, 

 is the presence of erratic blocks which may have been transported 

 by icebergs and dropped into the sea. But unless the glaciers 

 of that epoch reached the sea or the sea was frozen, we could not 

 possibly have even this evidence. Traces in the stratified rocks 

 of the effects of land-ice of former epochs must, from the very 

 nature of things, be rare indeed. The only sort of evidence which, 

 as a general rule, we may always expect to find is the presence of 

 large blocks of older rocks found imbedded in strata which, we 

 know from their constitution, must have been formed in still water. 

 But this is quite enough ; for it proves the existence of ice at the 

 time that the deposit containing the blocks was being formed as 

 conclusively as though we saw the ice floating with the blocks 



