﻿1851.] 



TRIMMER ON ERRATICS OF CHESHIRE. 



205 



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small scratched detritus is characteristic of 

 the Lower deposit, or Till. I have not ob- 

 served an instance of it in the Upper Er- 

 ratics, either of the east or west of Eng- 

 land. From recent observations, I doubt 

 whether these scratches are found on the 

 smaller fragments of granitic and other 

 northern rocks in the Till itself, which 

 have been derived from great distances, and 

 whether they are not confined to such as 

 have come from the neighbouring high 

 grounds. Should this prove to be the case 

 generally, it would throw much light on the 

 history of these deposits, if we could deter- 

 mine the exact nature of the agencies by 

 which the polishing and scratching of these 

 fragments were effected, whether they are 

 to be referred to the action of marine, or 

 of terrestrial ice. When slabs of slate are 

 found, as I have seen them in the valleys of 

 the interior of Wales, with one side po- 

 lished and scratched, the other remaining 

 quite sharp, we can have no difficulty in 

 supposing the upper surface to have been 

 protected, by being fixed in a mass of ice. 

 When both sides have been polished and 

 scratched as in the specimen of limestone 

 from the Till at Manchester, now exhibited 

 to the Society (see fig. 3, page 207), it may 

 be supposed, that, having been imbedded in 

 ice, its under-surface was scratched in pass- 

 ing over hard points, and that, having been 

 frozen-in during another season in a reversed 

 position, the same process was repeated on 

 the opposite side. It is objected, that, if 

 the ice were shore-ice, the first set of 

 scratches would be effaced by the action of 

 the sea on the beach before the stone could 

 be again enveloped in ice. It may be sup- 

 posed, on the other hand, that, the stone 

 being stationary, the ice, having small gra- 

 vel and sand frozen into it, passed over the 

 stone, polishing and scratching the upper 

 surface ; and that, the position of the stone 

 having been reversed, the opposite side was 

 subsequently subjected to the same action. 



The Narratives of the Polar Voyages fur- 

 nish many facts illustrative of the manner 

 in which fragments of this kind might be 

 imbedded, under an arctic climate, in a lit- 

 toral deposit of mud. 



