SHRINKAGE FISSURES OLDER ROCKS. 29 



stratified with sedimentary rocks; and Kenwood, 

 when describing East Cornwall, states that in 

 Mentreniat, " the whole strata seem broken up by a 

 succession of disturbances of a nature between cross- 

 courses and slides." Somewhat similarly a slide 

 may graduate into a lode, for in alternations of " soft 

 and hard ground," such as shales and grits, the lode 

 will widen in the former, while it may " choke out " 

 entirely in the grits, and only form a simple single 

 break. Cross-courses and lodes also merge into one 

 another, the fissures having been in part filled with 

 fault-rock, and in part by a mineral vein, or by a 

 combination of both ; the interstices in the fault-rock 

 having been filled with minerals deposited from 

 solution. Kenwood mentions a remarkable example 

 at Mentreniat, 2 and such are not uncommon in other 

 localities. 



Most of the shrinkage fissures beneath the surface 

 of the earth seem to be filled with either fault- 

 rock, or crystalline mineral matter, but those formed 

 at the surface are affected more or less by the 

 atmospheric agencies. If surface fissures occur in 

 level ground, or in ground from which there is only 

 a slight fall, meteoric abrasion will more or less 

 obliterate them ; but if they are on a slope, or there 

 is a subterranean drainage from them, the meteoric 



1 " Cornwall Geological Transactions," vol. viii., p. 714. 



2 Ibid., p. 713. 



