SHRINKAGE FISSURES OLDER ROCKS. 21 



due to weathering. Nevertheless, the peculiar character 

 of a large proportion of joint-faces seems to indicate 

 that something more than mere cracking consequent 

 on shrinkage has been concerned in the formation of 

 them. This is perhaps particularly well illustrated 

 in granite, and also indeed in the hard conglo- 

 merates. The way in which the joint-planes often 

 cut smoothly and evenly through the matrix and the 

 quartz pebbles in such conglomerates, is a perfect 

 mystery, and not to be explained by mere cracking 

 or mere fracture of any kind, as the fracture-surfaces 

 of these rocks is often totally different in character 

 from the ordinary joint-surfaces. Haughton suggests 

 that these surfaces are due to an obscure tendency 

 to cleavage having previously existed in the rocks. 



Our colleague, the late F. J. Foot, M.A., paid 

 particular attention to the jointing of the lime- 

 stones in the barony of Burren, Co. Clare, and of it 

 writes : 



" The main joints, in this district, range between 

 N. 45 W. and N. 45 E., the most prominent being 

 those from N. 5 E. to N. 10 E. These may often be 

 seen running in perfectly straight lines for several 

 miles. Sometimes the piece of rock between two of 

 these is traversed by numerous small parallel joints, 

 extending for the same distance, and cutting up the 

 rock into vertical laminae. The action of the weather 

 on these will in time wear away this intervening 



