Geological Society of London. 285 



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 (h) structures w^hich 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 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 

 sufficiently to soften them. 



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

 Mexico." By Frank Eutley, Esq., F.G.S. 



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

 The sequence of the structures developed in it is made out to be as 

 follows : — First, the development of fluxion-banding ; next, the 

 formation of spherulites ; and then the setting up of a perlitic 

 structure, the fissures of which were finally sealed by the intro- 

 duction of chalcedonic matter. 



A wavy transverse banding in the spherulites is apparently due 

 to a temporary check which the fluxion-bands have exerted on the 

 development of the crystalline bundles of the spherulites. In one 

 case a spherulite has been developed prior to the formation of a 

 similar but larger one which encloses it. Some of the spherulites 

 envelope small crystals of triclinic felspar. 



The author considers it very probable that the obsidian has been 

 subjected to hydrothermal agency since its solidification, and sub- 

 sequent to the development of its perlitic structure, and gives 

 reasons for this view. 



III.— May 6, 1891.— Dr. A. Geilde, F.R.S., President, in the 

 Chair. — The following communications were read : — 



1. " On a RhEetic Section, at Pylle Hill or Totter Down, Bristol." 

 By E. Wilson, Esq., F.G.S. 



In a deep railway-cutting at Pylle Hill, the Rhsetic beds, having a 

 thickness of not more than seventeen feet, are exposed between the 

 Tea- Green Marls and the Lower Lias. There is no doubt as to the 

 division between the Rheetic and Keuper beds in this section, but 

 the line of demarcation between the Rhajtic and the Lias has always 

 been a matter of uncertainty in the West of England. In con- 

 nection with this subject the term " White Lias," as applied to beds 

 some of which are Rhastic and others Liassic, is held to be unsatis- 

 factory. The author takes a limestone which is the equivalent of 

 the Gotham Marble as the highest Rhgetic bed in the section 

 described. Pie divides the Rhaetic beds of the cutting into an 

 Upper-Rhgetic Series and Avicida-contorta Shales. The intimate 

 connexion betwixt the Tea-Green Marls and the Red Marls of the 



