ON THE CKYSIALLINE ROCKS Ol' TUH LIZAUl) UISTRlCT. 400 



olivine rock, of igneous orijj^in ; and, secondly, the ophicalcite, con- 

 sisting of calcite and serpentine intermixed and generally banded. 

 He was glad that the Authors maintained the original igneous origin 

 of the Cornish serpentine, which most resembled the former variety 

 in the West of Ireland. With regard to the l>aiiding of the dykes of 

 gabbro where they were in contact with the walls, he observed that 

 this was a structure not uncommon amongst dykes of igneous rock, 

 and he believed it to have originated during the cooling process. 

 These bands were, in fact, planes of cooling, and the structure of the 

 rock along the walls of the dyke contrasted with the central portions, 

 where the cooling process was slower, and allowed the formation of 

 a more crystalline rock in which those planes were absent. 



The Pkesiuent remarked that the questions discussed in the paper 

 had far more than a mere local interest. In particular, the problem 

 of the banded structures among crystalline schists touched some of 

 the profoundest difficulties of the theory of metamorphism. There 

 was ground, he thought, for believing that mechanical deformation 

 had been rather too freely appealed to as an explanation of the 

 general banded and schistose structures of the older rocks. This 

 cause had unquestionably been largely instrumental in the pro- 

 duction of such structures ; but, as he had stated in his Anniversary 

 Address, there were features of the more ancient gneisses which it 

 was hard to imagine could be due to anything else than some original 

 variations in the arrangement of the materials of the rock before soli- 

 dification. He had been much struck with the extraordinary Avay 

 in which some of the Tertiary gabbros of Skye simulate the rudely- 

 parallel wavy lenticular banding of different materials in many 

 gneisses; and he thought it was rather among such examples of flow- 

 structure in eruptive rocks that the analogies of some of the struc- 

 tures of the gneisses were to be sought. The Authors had, there- 

 fore, in his opinion, done a service in recalling the attention of 

 geologists to this view of the subject. 



General M'^Mahox said that, as those who had taken part in the 

 debate appeared to agree generally with the couclusions arrived at 

 by the Authors, he would coufine himself to calling special attention 

 to one of the specimens exhibited, and to a brief description of a 

 section which he thought had an important bearing on the subject 

 under discussion. 



Prof. BoNNEY stated that the theory suggested by Prof. Hull had 

 been, in substance, formerly held by himself, but that he had found 



, cases for which it did not suffice. The case quoted by the President 

 was of great interest, and he might add that since the paper was 

 written he (the speaker) had seen others. He could not sit down 

 without testifying to the value of Mr. Teall's work at the Lizard, 

 and begged the Society to remember that General INPMahon was the 

 originator of the right idea (as the speaker believed it to be) as to 



ithe foliation of the gabbro. 



Q.J.G.S. Xo. 188. 2n 



