324 Geological Society : — 



January 23. — E. Etheridge, Esq., E.E.S., Vice-President, 

 in the Chair. 



The following communications were read : — 



1. "On the Serpentine and associated Eocks of Porthalla Cove." 

 By J. H. Collins, Esq., F.G,S. 



In a paper read before the Eoyal Geological Society of Cornwall 

 in November 1879 the author described the rocks at Porthalla Cove 

 as consisting of much-contorted strata of slaty green and red ser- 

 pentine- and hornblende-rocks, which he regarded as " highly altered 

 Lower Silurian stratified rocks." Prof. Bonney has since (Q,. J. 

 G. S. vol. xxxix. p. 1) described the same rocks, and explained 

 the structure of the district quite differently, correlating the rocks 

 with those of his " Archaean metamorphic series " at the Lizard. 

 The author in the present paper argued in support of his previously 

 expressed opinion. 



He stated that within a space of little more than 100 yards the 

 following varieties of rocks may be distinguished: — 1. Crumpled 

 shales and slates, sometimes containing veins and layers of quartz 

 and flakes of mica ; passing into 2, greenish slates of talcose appear- 

 ance ; passing into 3, soft and shaly mudstone ; passing into 4, red 

 and green bands of serpentine ; often passing into 5, hornblende- 

 schists of the type characterizing the locality; and 6, pinkish or 

 greyish granulite. No. 1 he regarded as belonging to a younger 

 series than the rest ; nos. 2 and 6 are the rocks supposed to repre- 

 sent Prof. Bonney 's micaceous group of the Archaean series ; and 

 no. 4 was regarded by the latter as intrusive. 



The author stated that the slates, shales serpentine, and horn- 

 blende-schist appeared to him to be distinctly interstratified, and 

 the granulite to be distinctly intrusive. At Nelly's Cove, about 

 half a mile north of Porthalla, there are a few thin bands of black 

 limestone, like those of Gerrard's Bay and also containing frag- 

 ments of Crinoids ; these were regarded by the author as of Lower 

 Silurian age, and as underlying the Porthalla rocks considered by 

 Prof. Bonney to represent the Archaean metamorphic series of the 

 Lizard Head. The author discussed at considerable length the 

 arrangement and relations of these rocks, and gave chemical analyses 

 of many of them, dwelling especially upon the comparative analyses 

 of a " talcose slate " from Porthalla and of one from the Lizard, the 

 latter regarded as characteristic of Prof. Bonney's Archaean mica- 

 ceous series at that locality. The differences shown by these analyses 

 he considered incompatible with the identity ascribed to the two 

 rocks. The serpentine of Porthalla he regarded as produced from 

 the hornblende-schist (itself a metamorphosed Silurian rock) by an 

 extreme alteration caused by the action of the magnesian salts ex- 

 isting in sea-water, probably during a period of marine submer- 

 gence. Although regarding the granulite as well as the gabbro of 

 the district as intrusive, he thought that the existing contortions of 



