On a Spherulitic and Perlitic Obsidian from Mexico. 137 



authors have failed to discover any evidence, and consider it 

 contrary to stratigraphical and petrographical facts. 



In regard to the genesis of the crystalline schists, which for pur- 

 poses of reference were divided by Prof. Bonney into a " granulitic," 

 a " hornblendic," and a " micaceous" group, the authors show that 

 in parts of the first the more acid rock breaks through the more basic, 

 as if intrusive, in others they appear to be perfectly interstratified, 

 the one passing backwards and forwards, though rapidly, into the 

 other. But between these extremes, intervals can be found where 

 the two rocks seem as if partially drawn out together. The authors 

 are agreed that certainly one, probably both, of these rocks are 

 igneous, that when the basic rock was solid enough to be ruptured 

 the acid magma broke into it, and sometimes softened it sufficiently 

 to allow of the two flowing for some little distance together, after 

 which crystallization took place. In regard to the hornblende schists, 

 the authors are not yet satisfied that either fluxion or mechanical 

 crushing will account for every structure which they have examined, 

 and prefer to leave the question, in certain cases, an open one. The 

 most distinctive features of the micaceous group appear due to sub- 

 sequent earth- movements, so that, though it exhibits some special 

 characteristics, the authors are doubtful whether it is any longer 

 worth while separating it from the hornblende schists. 



Of the igneous rocks newer than the serpentine, the gabbro has 

 received the closest attention. It exhibits in places (especially in 

 the great dyke-like mass at Carrick Luz) a very remarkable foliation 

 or even mineral banding, which has been claimed as a result of 

 dynamo-metamorphism. The authors bring forward a number of 

 instances to establish the following conclusions : — {a) That this folia- 

 tion occurs most markedly where the adjacent serpentine does not 

 show the slightest sign of mechanical disturbance ; (6) that it must 

 be a structure anterior to the consolidation of the rock ; (c) that it 

 sets in and out in a very irregular manner : (d) that when it was 

 produced the rock was probably not a perfect fluid. Hence they 

 explain it also as a kind of fluxion structure, produced by differential 

 movements in a mass which consisted of crystals of felspar and 

 pyroxene, floating thickly in a more or less viscous magma. 



The authors' investigations tend to prove that (a) structures 

 curiously simulative of stratification may be produced in fairly 

 coarsely crystalline rocks by fluxional movements anterior to crys- 

 tallization ; and that (b) structures which of late years have been 

 claimed as the result of dynamo-metamorphism subsequent to con- 

 solidation must have, in many cases, a like explanation. This is 

 probably the true explanation of a large number of banded gneisses 

 which show no signs of crushing and are holocrystalline, but in their 

 more minute structures differ from normal igneous rocks. 



The authors have seen nothing which has been favourable to the 

 idea that pressure has raised the temperature of solid rocks suffi- 

 ciently to soften them. 



2. " On a Spherulitic and Perlitic Obsidian from Pilas, Jalisco, 

 Mexico." By Prank Rutley, Esq., P.G.S. 



The specimen described is a leek-green rock with waxy lustre. 



