Reports and Proceedings — Geological Society of London. 91 



Geological Society of London. 



L— December 15, 1897.— Dr. Henry Hicks, F.R.S., President, 

 in the Ciiair. The following communications were read : — 



1. " On the Pyromerides of Boulay Bay, Jersey." By John 

 Parkinson, Esq., F.Gr.S. 



After briefly noticing the literature of the subject, the author 

 describes the altered rhyolites of Boulay Bay. One variety, the 

 commonest, is of a dark-red colour, showing flow-structure ; another 

 is porphyritic ; a third, near the centre of the Bay, has a pale- 

 greenish matrix enclosing fragments, which, however, are due to 

 flow-brecciation. Large pyromerides occur in two localities ; in 

 the more interesting, that north of the jetty, the structure of the 

 rock indicates either a very peculiar magmatic diffei'entiation in situ 

 or (more probably) the mixture of two magmas differing in their 

 stage of consolidation. 



From study of a series of specimens of the pyromeridal rock, the 

 author arrives at the following conclusions : — (1) The rock shows 

 marked flow-structure and at times bands which indicate a slight 

 difference in its composition, the latter tending to assume a monili- 

 form outline. In such the microscopic structure corresponds with 

 that of the pyromerides, and exhibits traces of radial crystallization. 



(2) These afford a passage into somewhat oval pyromerides, 

 with rather tapering ends and irregularly mammillated surfaces. 



(3) From these sometimes a single one seems to be thrown off, 

 while lines of pyromerides or little lumps of similar material are 

 scattered about the matrix. (4) Many of the pyromerides are solid 

 throughout ; others have a central cavity filled with quartz. 



The author describes varieties of the pyromerides. They are 

 generally deep-red in colour, and exhibit (a) fluxion-structure, made 

 more distinct by minute black microliths ; (&) a radial structure ; 

 (c) a "patchy" devitrified structure (with crossed nicols) ; — the 

 second (h) being not always present. The matrix is usually of 

 a greenish tint, showing devitrification-structure and sometimes a 

 trace of perlitic structure. 



The pyromerides frequently exhibit more or less crescentic cracks, 

 due apparently to contraction, which have been filled by quartz. 

 Sometimes also they scale off in rudely crescentic shells. In one 

 locality a variety with good spherulites, about as large as a pea, 

 passes into one showing a fluxion-structure and pyromerides, having 

 traces of radial structure as well as clots and irregular " wisps," 

 suggestive of a stiffer material broken up by one more liquid. 



As the result of his studies, the author thinks that while very 

 regular spherulites do occur, apparently in consequence of radial 

 crystallization round a centre, the pyromerides are due to the mixture 

 of two magmas slightly different in composition and fluidity, the less 

 plastic of the two being sometimes drawn o>it into streaks, but at 

 others forming lumps, in which, where their form is suitable, a radial 



