" greenstones " op western cornwall. 179 



Discussion. 



Mr. Sorby stated that he had come to nearly the same conclusions 

 with Mr. Phillips with respect to these rocks. 



Dr. C. Le Neve Foster agreed with the author in considering 

 some of these so-called greenstones altered rocks, and not intrusive, 

 as had been supposed. He congratulated geologists on having such a 

 combination of chemical and microscopical analysis as that furnished 

 by Mr. Phillips. 



Mr. Kutley thought that many of the bands might be identical 

 with those occurring in Devonshire, especially near Tavistock. The 

 dipping away from the granite described by the author might be 

 due to the granite having been subsequently intruded. He thought 

 it probable that many of the beds referred to may be ash-beds, and 

 that it was quite possible the schistose character might be super- 

 induced in beds of ashes by the pressure of superincumbent rocks. 

 The mapping of Sir Henry De la Beche is remarkable for the accuracy 

 with which these boundary-lines are drawn ; furthermore his deter- 

 minations of these rocks, as shown in his report, are either absolutely 

 correct or close approximations to the truth, although the small scale 

 of the map does not permit this to be shown. 



Mr. Koch remarked that, in considering the nature and origin of 

 such rocks as these, we ought to notice what rocks overlie them, 

 and to take into account the probable effects of water percolating 

 through such overlying beds and containing much carbonic acid, 

 by deposition of silica and removal of carbonate of lime. The 

 analysis of the killas given by the author was almost that of altered 

 trachyte. He thought that in such investigations the microscope 

 must be our principal guide, but aided by the consideration of the 

 relative positions of the rocks. 



Prof. Ramsay inquired what was the definition of a volcanic rock. 

 He thought that volcanic rocks were either lavas or ashes, and that 

 it was dangerous to affirm that the blue elvans were ashes in so 

 metamorphic a district. 



Prof. Maskelyne, after thanking the author for his paper, 

 inquired as to the meaning in which he used the terms " gabbro " 

 and " igneous rock." He also remarked that it was strange to find 

 apatite the only mineral that was unaltered. 



Mr. Drew inquired as to the possible future colouring of the 

 Survey Map of this district. It appeared from the author's paper 

 that the parts coloured as " greenstone " in the present maps 

 included various rocks which ought to be subdivided. 



The Author, in his reply, stated that igneous rocks have a certain 

 amount of cleavage, which is often rendered very distinct by 

 weathering. The crystals of hornblende are very distinct in the 

 rocks described. Gabbro is a felspathic rock with diallage, sma- 

 ragdite, or hypersthene instead of augite : but with regard to some 

 of them it may be doubtful whether they are gabbros or dolerites. 



