SHRINKAGE FISSURES OLDER ROCKS. 27 



forced up from below or dropped into the fissure from 

 above ; while a lode is a shrinkage fissure, or a fault 

 now filled with minerals and their associates. The 

 fault-rock generally found in cross-courses would 

 appear to be due to the rubbing up and down of one 

 wall of the break against the other, as happens with 

 the sides of a crack formed in a wall during an earth- 

 quake ; and in general the strata on either side are 

 displaced from their original position. This, however, 

 is not always the case, as in some instances the beds 

 of rock on both sides would appear to have returned 

 to their first positions, cross-courses being known to 

 cross lodes, and yet neither to heave nor slide them. 

 Some of these cross-courses may possibly have been 

 open fissures that were filled from above, but others 

 could scarcely have been so. 



Most lodes, originally, were fissures due to the 

 shrinkage of the rocks. These eventually were filled 

 by minerals deposited from solutions ; and, as just now 

 stated, it is probable that in many cases the fissures 

 enlarged gradually. At Glandore Mine, Co. Cork, 

 there are three distinct formations occurring together, 

 and forming one great " mineral channel; " namely, 

 manganese-ores, hematite and fault-rock ; the latter 

 being the oldest, as the hematite occurs in a lode 

 that crosses and sends veins into the fault-rock, while 

 veins of the manganese - ores are found cutting 

 through both. 



