Geological Society of London. 187 



portion of the Eed Crag, but that whilst many species having a 

 southern character are present at St. Erth, and wanting in the Crags 

 of the East coast, the Boreal and Arctic forms found so abundantly 

 in the Crag are absent at St. Erth. 



In explanation of this remarkable fact, it is suggested that when 

 the St.-Erth beds were deposited, although the North-Sea area was 

 in direct communication with the Arctic Ocean, the western part of 

 the British Channel was not, that the British Isles were joined to the 

 continent of Europe on one side and to Greenland on the other, the 

 Shetland and Faroe Islands and Iceland being the remnants of 

 the barrier that formerly divided the Atlantic from the Arctic Sea. 

 Evidence is given in support of this view from the present sub- 

 marine configuration of the North Atlantic. It was also shown to 

 be probable that the St.-Erth area, in Pliocene times, was more 

 directly connected with the Mediterranean than at present, by a 

 marine channel that traversed France. 



III.— March 10, 1886.— Prof. J. W. Judd, F.E.S., President, in the 

 Chair. — The following communications were read : 



1. " On the Alteration of coarsely spherulitic Rocks." By Gren- 

 ville A. J. Cole, Esq., F.G.S. 



The author stated that the pitchstone of Zwickau, Saxony, contains 

 large spherulites, remarked on by Cotta in 1847, which are devoid 

 of radial structure, and are traversed by the fine lines of flow that 

 characterize the glassy matrix. The centre of each of these has been 

 hollowed out by decomposition along cracks, as may be seen at the 

 ends of the branches into which the cavity divides,. and infiltration 

 of chalcedony and calcite has occurred. The lines of flow corre- 

 spond on opposite sides of this secondary mass, and do not bend 

 round as if the spherule had formed about some calcareous inclusion 

 or about a vesicle. Similar excavation and infiltration have occurred 

 extensively among the coarsely spherulitic layers of the Precambrian 

 rhyolites of Lea Rock, Wrekin, and in these the cracks traversing 

 the rock are seen, under the microscope, as mere lines when passing 

 through the matrix, but widen out at once probably through more 

 ready decomposition along their walls, when they enter spherulitic 

 matter. In the white rock (Silurian rhyolite) of Digoed, near Pen- 

 machno, N. Wales, similar alteration has converted the closely set 

 spherulites (often three inches in diameter) into mere shells filled 

 with quartz and chlorite; while a black slate-like decomposition- 

 product, with a hardness of 2-5 and a specific gravity of 2*77, occurs 

 occasionally here, and very frequently in the less coarse but similar 

 rock of Conway Mountain. In the latter place spherulites may be 

 found containing this product in alternate concentric layers. Analysis 

 shows it to be allied to pinite ; but the formation in it, when it tends 

 to become crystalline, of microlites of various kinds, opposes its 

 being regarded as a simple mineral. The rock of Digoed itself 

 contains 83 per cent, of silica, the black product only 50 per cent. 

 The great nodular masses of quartz and these contrasted soft, black, 

 cleavable layers form striking features of alteration, especially as 



