Quartzytes a7id Quartz- Rocks. 163 



matrix; but necessarily in consequence of their origin and 

 mode of deposition, their structural lines will not coincide with 

 those of their matrix. Therefore, any microscopist who may 

 form a theory solely on their appearance is altogether at sea. 

 As far as my experience goes, such inliers in quartz-rock 

 are so similar to the matrix, that in the majority of cases ordi- 

 nary field workers would not detect them, the inliers being 

 quartz-rock, and the matrix quartz-rock. I know, however, 

 exceptions where the inliers are much more felsitic, or ferri- 

 ferous, or calcareous than the matrix ; or the inlier may be a 

 perfectly different class of quartz-rock to the matrix, thus 

 giving to the rock a fine conglomeritic appearance. This I 

 especially noted in a mass of quartz-rock in the Co. Donegal, 

 to the west of Mulroy Bay. 



Such fragments, I would suspect, came up from below, as 

 fragments will come up in springs when in violent action. 

 Thus a strong spring in a limestone tract may, when w^orking 

 violently, bring up from a deep source silicious sand. 



These fragmentary quartz-rocks, if only examined micro- 

 scopically, may be pronounced to be clear proofs of the origi- 

 nal sedimentary origin of the rock; but if the mass of the rock 

 is examined one has to come to a different conclusion. 



Fragmentary portions of undoubted intrusive rocks are not 

 uncommon, as has been pointed out over and over again in 

 previous writings, and such rocks are much more complicated 

 than the brecciated quartz-rocks, as such intrudes usually con- 

 tain distinctly foreign inliers, while the quartz-rock rarely 

 does; although in places, as at the Eagle's Rock Hackets- 

 town, Co. Carlow, the intrudes of quartz-rock contain inliers 

 of granyte. As to "undoubted bedded lines" in the quartz- 

 rock of the Sugar lyoaves, Co. Wicklow, I do not believe that 

 such is their origin. The lines do exist, but they are not due 

 to original bedding, as will be proved if we go further afield. 



Similar lines, very generally, are ordinary characteristics of 

 quartz-rock. Numerous places could be mentioned all over 

 Ireland, but the following need only be specially referred to. 

 The intrude of quartz-rocks (** white rocks,") in the granyte a 

 little north of Tinnahely, Co. Wicklow, has in it similar lines ; 

 as also the intrude, a little further north, called the Eagle's 

 Rock, Hacketstown, Co. Carlow ; similar lines also occur 

 to the south- w^est of the Co. Wexford, in the dykes that extend 

 from the Oldhamian into the Ordovician. In the museum 

 of Trinity College there ought to be a specimen of a similarly 

 lined quartz-rock that was procured from an intrude at the 

 hill summit to the south-west of Oughterard, Co. Galway. 

 Examples could be multiplied so as to fill the whole of this 

 number of the Magazine. It is, therefore, only necessary to 

 say that such bedding-like lines are common in intrudes of 

 quartz-rock, but their exact origin has still to be determined. 

 In the quartz-rock which is the basal bed of the Carboniferous 



