SoLLAs — On the Structure and Origin of Quartzite Bocks. 171 



pressure into the overlying slates, which, it penetrated like an 

 igneous intrusion, sometimes running between the bedding planes 

 of adjacent beds of slate, so as to deceptively appear as though in 

 its original position ; but at length, when traced far enough, cutting 

 across them, and so revealing its truly intrusive character. 



On the occasion of the second visit of the British Association 

 to Dublin in 1878, Mr. Close (5) drew up an account of the 

 geology of the district in which he states that, though " generally 

 conformable," the quartz rocks " sometimes assert their indepen- 

 dence of the stratification in a way that is not easy to explain." 



Mr. Kinahan (6) pushed the argument in favour of an eruptive 

 origin of the quartzites a step farther ; and in his Greology of 

 Ireland they are treated under the heading of eruptive rocks. In 

 the preface to this work (p. vii) they are termed quartz rock to 

 distinguish them from other recognised varieties of quartzite or 

 quartz-schist. In a footnote on the same page Mr. Kinahan adds : 

 " In the counties of Wicklow, Dublin, and Wexford I have found 

 quartz rock in unaltered Cambrians, in the county of Wexford in 

 unaltered Cambro-Silurians, in the county of Mayo in unaltered 

 Silurians, and in the counties of Clare and Gralway in the Old Red 

 Sandstone, and these facts alone ought to demonstrate that quartz 

 rock should not be included among the metamorphic rocks, and 

 that it is a distinct rock from quartzite or quartz schist, the latter 

 being a true metamorphic rock." 



Again, on page 194 {loc. cit.), we find the following: — "In 

 the Cambrians at and to the S.W. of Wexford town (Forth 

 Mountains) are dykes and cake-like masses of quartzite that are 

 [?not] metamorphosed quartz rocks. Such cakes of quartz rock 

 appear to have been deposited from springs or some such accom- 

 paniment of feeble vulcanicity, the cakes being surrounded and 

 eventually covered up by the deposition of the contemporaneous 

 sedimentary rocks, while the dykes mark the sites of the passages 

 through which the mineralized waters found egress. To the north- 

 ward and eastward of the Cambrian rocks are other masses of quartz 

 rock in the Cambro-Silurians. These are especially numerous to 

 the south-westward near Bannow, where some of them are felsitic, 

 and graduate into rocks like petro-silex and fine elvan." Further 

 on, page 196, as follows: — "In the Cambrian rocks of the counties 

 of Wicklow and Dublin are protrusions, dykes, and cakes of quartz 



