ITS SECOND STAGE. 235 



down as clay and mud, mounds of shingly gravel, and 

 masses of blocks and boulders. Whatever the nature of 

 the parent rock, these mounds and masses would partake 

 of it yellow clays and schists and granites in granite dis- 

 tricts, red clays and red sandstone blocks in Old Eed 

 Sandstone areas, and dark -coloured clays and blocks of 

 limestone and sandstone in Carboniferous basins ; and as a 

 general rule these blocks and boulders not far transported 

 from the cliffs and precipices from which they had been 

 torn by the ice -giant. Indeed, in most instances, this 

 proximity of glacial clays, ice- worn and ice-scratched blocks, 

 is one of the best proofs of the first stage of the ice-epoch, 

 and all over the northern and middle portions of Scotland 

 we have never found it to fail in its indications. 



Eut as the cold set in more intensely, the downward 

 movement of the land seems to have commenced ; and 

 hence much of the ice-worn debris of this first stage was re- 

 moved by denudation, step by step, as the terrestrial surface 

 descended. "What may have been the precise character of 

 the climate at this epoch of descent that is, how long the 

 winter frosts and how short the summer's thaw we have 

 no means of determining, for the earlier clays and moraine 

 blocks are destitute of organic remains ; but if we may 

 judge by the comparatively small amount of obliteration by 

 denudation, it would appear that the seas were more ice- 

 locked than free-flowing, and that consequently the land 

 went down encased, as it were, in ice of prodigious thick- 

 ness.* This descent or subsidence of the land forms the 



* The ice-sheet at this stage may have been two or three thousand 

 feet in thickness. The great antarctic ice-barrier, met by Sir James Ross 

 and his companions, was estimated at a thousand or fifteen hundred feet. 

 Ice of this thickness would rest on the beds of all the shallower friths and 

 seas, and act upon them precisely as upon the rocky surfaces of the dry 

 land. This circumstance should be carefully borne in mind in reasoning 

 on the phenomena of the glacial epoch. 



