28 SHRINKAGE FISSURES OLDER ROCKS. 



That class of granite vein for which Dr Sterry 

 Hunt has proposed the name of " endogenous veins," 

 is evidently allied to mineral lodes, as the rocks 

 now filling them were deposited from solution. This 

 is evident, as most of them are lenticular, that is, 

 die out in every direction ; consequently they could 

 not have been filled by matter injected into them. A 

 character possessed by them, in common with mineral 

 lodes, is, that in many of them there are successive 

 layers or ribs along the walls, usually of very similar 

 minerals, but always differing in texture. That such 

 layers, filling shrinkage fissures, have been deposited 

 successively, can in some instances be proved by veins 

 and strings branching from the inner layers, not only 

 into the outer layer, but sometimes even into the 

 adjoining rocks ; as if the more recent shrinkage had 

 affected, not only the rock mass, but also those parts 

 of the vein-rock that had been previously formed. As 

 slides, heaves, cross-courses, and lodes are closely 

 allied, one may graduate into another. The dyke of 

 fault-rock in a cross-course, or indeed in any fault, 

 may widen or narrow. If the rocks, through which 

 the fault passes, are alternations of hard and soft 

 strata, there will probably be a dyke of fault-rock in 

 the latter, while in the former its course may only 

 be marked by a line or mere parting; a cross- course 

 graduating into a slide or heave. This is often 

 found to be the case when igneous rocks are inter- 



