182 Scientific Proceedings, Royal Dublin Society. 



the occasional occurrence of a quartz grain different in character 

 from the rest ; thus, sometimes one finds a grain of exceptional 

 clearness and transparency bounded by rounded outlines, and 

 surrounded on all sides by the dusty grains which compose the 

 mass of the rock ; sometimes, on the other hand, a peculiarly 

 " dirty " grain may be met with, in equally striking contrast to 

 its clearer neighbours, or one in which long prisms (of cyanite?) 

 occur, its neighbours being all devoid of them. Such contrasts as 

 this do not occur in the true quartz mosaics of granitic rocks. 



Of additional minerals I have only observed an occasional 

 rhombohedron of iron-carbonate and a few minute cubes of iron- 

 pyrites, and these are without bearing on the question of origin. ' 



I admit that this is a difficult rock to explain in all its details, 

 but the preponderance of evidence, derived from its microscopic 

 characters alone, appear to me to lie wholly in favour of its clastic 

 origin. 



This conclusion is in harmony with the results obtained from 

 a study of the Dublin quartzites, where the evidence is of the 

 clearest nature, and since the general mode of occurrence of the 

 rock is the same on both sides of the channel this is a result we 

 might naturally have expected. 



Lamination. — The quartzites of our neighbourhood frequently 

 exhibit in the field a curious striping, which is particularly obvious 

 on weathered faces ; the stripes are very narrow bands, from about 

 a twentieth to an eighth of an inch in breadth, of a slightly darker 

 colour than the rest of the rock, than which they weather some- 

 what more rapidly. To the unaided eye they suggest planes of 

 original deposition, and such I shall show they undoubtedly are. 

 In the quartzite quarry on Oarrickgologan, where I first pointed 

 them out to Professor Blake, I searched long for some planes 

 having a definite direction, which might give one some idea of 

 the stratigraphical position of the rock ; joint planes preserving a 

 general direction for considerable distances there are, which most 

 deceptively suggest bedding, but these do not correspond with the 

 direction of the dirt bands (as we may term them), which are less 

 conspicuous features : once seen, however, there is no difficulty in 

 tracing them over considerable distances, and in the Oarrick- 

 gologan quartzite I find them maintaining a strike with great 

 constancy over the whole ridge in a direction which is almost 



