EY HUGH MILLER. 32 1 



"When it is considered that limestones are subjected both to me- 

 chanical disintegration and chemical solution, it may be some 

 matter for surprise that this should be the case. Jukes likens 

 the limestone rocks of the South of Ireland to " glaciers melting 

 in the summer sun." There can be no doubt, however, that this 

 comparison is overstrained, unless taken in a general sense with 

 reference to the principles illustrated by Jukes when he used it. 

 T»J'either in the South of England nor in Yorkshire are the carbo- 

 niferous limestones conspicuously more liable to weather than the 

 sandstones ; in one case at least, when placed side by side with 

 Millstone Grit, in the walls of Carnarvon Castle, Prof. Eamsay 

 mentions their superior durability.** 



The shape of the limestone features in this district makes the 

 fact at once apparent that they are not moulded by chemical ac- 

 tion. "While the tendency of the mechanical agents is to use 

 their vantage in front by driving back the scarp, chemical solu- 

 tion tends to dissolve it down and convert the angular feature 

 into a rounded one. This is far from having taken place : lime- 

 stone features are generally as angular in section as those of 

 sandstone. 



Frequently, however, the effects of chemical action are suffi- 

 ciently well-attested in other ways. On the surface of a lime- 

 stone dipslope the joints are sometimes so widened by solution 

 of the sides of each block, that the slope displays a sort of check 

 pattern, — squares bounded by excavated lines ; a pattern which 

 the green-sward peculiar to limestones exhibits at certain seasons 

 of the year in shades of darker and lighter green like a tartan. 

 In quarries each of the upper line of blocks has generally a some- 

 what conical shape owing to solution of the sides. There does 

 not appear to be any reason to think that the surface of the dip- 

 slope has suffered much more from solution than the sides of any 

 one of these blocks, an amount represented by only a few inches. 

 Chemical action is distributed along the joints far into the depths 

 of the rock.f 



* Sec evidence gatheroa on this point by Mr. W. Giuin, Gool, Mug., 1876, p. 97. 



t Mr. Tlcldeman mentions erratic bloclis in Lancasliirc, supported on pedestals, one and 

 II lialf to two feet high, of the Carboniferous Limestone on which they wore stranded; the 



