186 Scientific Proceedings, Royal Dublin Society. 



white quartz, as is frequently the case, they give a very striking 

 appearance to the rock (PL xv., fig. 4). This is probably the 

 explanation of many " gash " veins. 



In many cases such fissures have been injected under the 

 mountain pressure by the material of the surrounding slate, and 

 have frequently also been simultaneously very much increased in 

 width, so that the originally continuous quartzose bed has become 

 converted into a stream of joint blocks. Grenerally a bending and 

 flowing of the sandstone has proceeded along with the other modes 

 of deformation, but evidently the pressure has been far from 

 uniform on each side of the moving stratum, so that great frag- 

 ments have been pushed out from the general bedding plane and 

 carried away by flowing argillaceous material, thus giving rise to 

 those strangely isolated joint blocks and the curious sudden trun- 

 cation of otherwise continuous quartzite beds. That scratches 

 and slickensides should generally characterize the joint planes 

 which bound the quarzite blocks is as naturally to be expected as 

 it is universally apparent. 



The apparently abnormal characters of the quartzite of our 

 district thus find an explanation. So far from being difficulties, in 

 the light of our present knowledge, they provide us with valuable 

 indications of the manner in which the deformation of a country 

 under earth pressures is accomplished ; had our Cambrian deposits 

 consisted wholly of plastic materials such as shales and slates, a 

 monotonous cleavage would have been the chief result, but thanks 

 to the presence of these less yielding quartz rocks we can study 

 with an infinity of detail the foldings, crumplings, faultings, both 

 horizontal and inclined, and the various other displacements which 

 make the headland of Howth the glory and despair of the geo- 

 logical mapper. 



P. S. — I reserve for a general account of the Hill of Howth 

 the description of certain gray and greenish grits which are com- 

 monly interbedded with the slates, but as some of my friends have 

 imagined that I included them in the general designation of 

 Cambrian quartzite, I may take this opportunity to disclaim such 

 an intention. These grits differ markedly from the quartzites, but 

 might easily be confounded by a hasty observer with the green 

 epidiorites (Lambay porphyry) which occur as such common 



