44 Dr Black on some appearances connected with the 



pare its disintegrated remains for supporting vegetation, but 

 I cannot think it could carry its disintegrating power to such 

 depths iii the solid stratified rocks, as observed in many of 

 these instances. It must also be remarked, that the surfaces 

 of these sandstones had, very probably, undergone more than 

 one geological catastrophe, or movement of abrasion, before 

 the diluvial one which now covers them with its remains from 

 the atmosphere ; for the new red sandstone was deposited sub- 

 sequently to the fractured elevation of these rocks, and was 

 most likely formed, in a considerable part, from the detritus 

 of these earlier secondary rocks. I have, therefore, been led 

 to attribute to the other external agent, namely, glacial conge- 

 lation^ the effects now under consideration. When we look 

 upon the common effects of frost upon porouls stones of a stra- 

 tified structure, or more particularly, if we observe the mode 

 in which the quarrymen prepare our coarse roofing sandstone 

 slates, we shall have little difficulty in understanding how the 

 first of the above operations took place. As water that has 

 percolated through rocks, or insinuated itself between the na- 

 tural layers of stones, expands so much on freezing, as to ex- 

 ert a force sufficient to dissolve their continuity, the quarry- 

 men avail themselves of this power by placing, during winter, 

 the entire laminated block from the delf on its edge, by which 

 the seams are better subjected to infiltration from the rain ; 

 and so, from repeated freezings and thawings, they are, by the 

 spring of the year, easily rent asunder into lamince of the na- 

 tural thickness which suits the purposes of slating in many 

 parts of the country. 



Infiltration, then, of surface-water and its consequent con- 

 gelation between the laminated strata of the rock appear to 

 have been the first agent in the work of breaking up and frac- 

 turing the superficial beds ; and, from the depth to which this 

 operation has in some places penetrated, it would appear that 

 the intensity of the congelating force had been considerable 

 for a short time, or else it had long continued in a more mor 

 derate degree, and most likely had been intermitted with 

 irregular or periodical thaws and fresh infiltrations. We have 

 next to account for the second part of the process, namely, 

 the mode in which the loosened and dissevered fragments were 



