26 SHRINKAGE FISSURES OLDER ROCKS. 



may not be due to their having been " successively 

 introduced, since all the substances may have been 

 in solution together, and circumstances having been 

 favourable at one time to the deposition of one sub- 

 stance and to that of another at another time." 1 



But this could not be the case in all lodes ; for in 

 some, such as the lode at Maumwee, West Galway, 

 the original lode stuff was metamorphosed contem- 

 poraneously with the associated rocks, while subse- 

 quent to that action a newer lode formed alongside 

 the older one. Other cases of subsequent contract- 

 ing are metalliferous strings and veins traversing 

 and cutting through the other minerals in a lode, or 

 a " rib " of ore may run for some distance along one 

 wall and then cross over to the other wall, evidently 

 occupying a crack due to shrinkage. 



Mining operations have taught us that there are 

 three distinct kinds of fissures or breaks. These are 

 called by the miners slides or heaves, cross-courses 

 and lodes. A slide or heave is a simple single 

 break in the strata along which the beds on one side 

 have slid down or been heaved up (fig. 7, PL I.) 

 A cross-course is a break, the walls or sides of which 

 are separated one from the other, while the inter- 

 vening space is filled with a dyke or mass ^fault-rock 

 (fig. 8, PI. I.), that is, a rock made up of fragments 

 and the detritus of the associated rocks, or of materials 



1 "Student's Manual of Geology," a New Edition, A.D. 1862, p. 359. 



