SHRINKAGE FISSURES OLDER ROCKS. 19 



seem to form when rocks are at or near the surface, but 

 some are deep-seated, as joint-lines may form at an 

 old surface, while subsequently newer strata may 

 have been deposited. In the old rocks these joints are 

 usually much more regular than in the recent accu- 

 mulations; nevertheless, in some beds of limestone 

 a jointing quite as irregular as, or even more so than, 

 that figured (fig. 1, PI. I.) may occur; and such 

 jointing, combined with subsequent weathering, will 

 change an even surface of limestone into a coarse 

 shingle, or into a number of irregular hummocks ; 

 each being surrounded by a deep hollow, and the 

 whole collectively having a somewhat similar appear- 

 ance to a tussocky bog ; both being due to the same 

 causes, namely, irregular jointing and subsequent 

 weathering. Jointing of this kind is not uncommon 

 in places among the limestone crags of Clare and 

 Galway; the interstices between these hummocks 

 varying from a few inches to yards in depth. We may, 

 however, remark that although we have mentioned 

 these localities as places in which these joints may 

 be found, yet they are not characteristic of the lime- 

 stone, for generally the jointing is extremely regular. 1 

 A barrel of Koman cement, if left in contact with the 

 atmosphere, will become indurated into stone. Such 

 an artificial rock was used as the corner-stone of a 



1 "Mem. Geol. Survey, Ireland," ex. sheets 85, 95, 105, 106, 113, 

 114, 115, 122, 123, &c. 



