430 ON CERTAIN ERUPTIVE ROCKS OF NORTH WALES. 



large -grained varieties is therefore very interesting, as is also the 

 determination of the "kind of felspar associated with the silica. The 

 paper was a very important contribution to our knowledge of cer- 

 tain rocks which possess much interest both mineralogically and 

 commercially. 



Prof. Ramsay said that he was glad to find that Mr. Phillips 

 maintained the broad general views originally set forth by the 

 Geological Survey. He presumed that the author considered the 

 Penmaenmawr rock not to be now, either chemically or minera- 

 logically, by any means in its original state of consolidation from 

 igneous fusion ; and he would be glad to know whether any conclu- 

 sion could be arrived at as to what this state was. He thought its 

 metamorphism might have been assisted by nearness of the rock to 

 the surface, favouring the percolation of surface-water. Continual 

 changes had probably been going on in the Penmaenmawr rock ever 

 since its original formation. He thought that the presence of such 

 large quantities of iron and manganese in the ash, as shown by the 

 author's analyses, might be due to infiltration rather than to these 

 substances being original constituents of the rock. 



Prof. Judd called attention to the other rock not noticed by the 

 previous speakers, whose remarks were confined to two only out of 

 the three sorts referred to in the paper. He remarked that the 

 uralite-porphyry of North Wales was now for the first time tho- 

 roughly investigated, so that we can now compare it with the uralite- 

 porphyry of Prcdazzo, which appears to be an altered augite-por- 

 phyry. The Welsh rock, in the changes round the edges of crystals 

 and in its appearance to the naked eye, differs from the Predazzo 

 porphyry. In the former much more of the characters of the augite 

 crystals is lost. Such careful descriptions and analyses as those 

 given by the author are most important and of the greatest interest 

 to English geologists. 



The Author, in reply to Prof. Earn say's question as to the ori- 

 ginal state of the rock at Penmaenmawr, said that the least-altered 

 rock was in the quarries opposite Beaumaris, where it exhibited 

 silica, hornblende, and triclinic felspar. It was remarkable that, 

 although in metamorphism free silica appears, the chemical compo- 

 sition of the rock is not altered. 



