KiNAHAN — Quartz, Quartz-Rock, and Quartzite. 585 



sion they should be placed. It must, however, be remembered that if 

 quartz-rock is sheared, it takes on a character similar to quartzite, 

 as is also the case in regard to granite and gneiss, as granite by shear- 

 ing changes into gneiss. It seems to me, therefore, that when quartz- 

 rock graduates into quartzite it may be due to a mass of quartz-rock 

 having been subjected to a shearing process which altered its margin 

 or margins, but left the main mass in its original condition. This is 

 suggested by the fact that the alteration is principally found in con- 

 nection with the planes of upthrust.' 



Dykes of quartzite may occur, they being due to dykes of quartz- 

 rock being sheared, but a normal quartzite, that is, an altered sand- 

 stone or grit, rarely occurs as a dyke. Sometimes, however, such 

 dykes, like rocks, do occur, as has been recorded in the counties Mayo, 

 Galway, and Wicklow, they being due to silicious matter in solution 

 coming up through joint lines and silicifying the adjoining rock. 

 Remarkable triangular patches thus changed into quartzite occur in 

 the Mweelrea grits, county Mayo. 



SlJMMAET. 



The following characters are common to quartzites, quartz-rock, 

 and vein quartz : — All are highly silicious, they are more or less 

 clastic, and have a laminated or foliated or bedded structure. 



Other characters of quartz-rock and vein quartz are the following: — 

 They may occiir as " lay and lay," that is between the natural beds of 

 the country rock, or as intrudes, or as protrudes, or as dykes. 



Some sandstones and grits make very similarly to quartz rock, 

 they appearing suddenly as massive protrudes, or as lenticular cakes, or 

 as dykes of fault rock. Such sandstones and grits nearly always have 

 false bedding, while often they are margined by faults, as if they had 

 acted as groynes during the different earth movements. The latter is 

 also often a characteristic of quartz-rock masses. 



Ordinary sandstones and grits nearly always contain fragments of 

 various kinds, due to their constituents being derived from various 

 sources, while normal quartz-rock, on the other hand, is usually 

 made up of silicious particles (vide Note B, p> 586). 



^ In the basal Kilmacrenans conglomerate of the Knockanteenbeg outlier, south- 

 west of Gartan lake, Co. Donegal, the inlying granite blocks and fragments are 

 elongated in the portion of the rock immediately above the upthrust planes, while a 

 little higher up they are not so. 



