SHRINKAGE FISSURES OLDER ROCKS. 23 



cross ones N. of E., but they are never traceable for 

 any great distance. 



" The general dip of the surface on Innisheer is 

 S. 15 to 17 W., and the most prominent open joints 

 bear N. 17 E." 1 This authority ignores the denu- 

 dation by wind. This force, however, as hereafter 

 shown (pp. 79 et seq), must have materially assisted in 

 weathering these joint fissures. 



Joints may be very variously developed as to fre- 

 quency, for at one time the members of a system will 

 be wide apart ; while at other times they may be so 

 close as to cut up the rock into slate-like slabs. 

 Cleavage in rock masses is easily determined, but 

 the distinction between minor jointing, when con- 

 fined to a single bed, and cleavage, is often obscure ; 

 in fact, in some cases one phenomenon seems to 

 graduate into the other. 2 



The contraction of rocks is apparent in mineral 

 veins that traverse alternations of soft and hard 

 strata. If there has been no displacement of the 

 rocks (slide or heave) in either wall or "cheek," the 

 parts of the same divided bed will be found opposite 

 each other on each side ; and if there has been no 

 shift, there can have been no friction or rubbing to 



1 "Mem. Geol. Survey, Ireland," ex. sheets 114, 122, and 123, pp. 

 21, et seq. 



2 W. King, D. Sc. Prof, of Geology, Queen Coll., Galway, has studied 

 the connection between these two phenomena. To his papers on the 

 subject the readers may be referred (Report Brit. Assoc. 1858, p. 83. 

 Scientific Opinion, 1870). 



