136 Geological Society: — 



following supplied material for the agglomerate overlying the tuff. 

 Lava then welled forth, and finally the volcano hecame extinct, and 

 the intrusive mass of the Stack, regarded "by the author as a 

 volcanic neck, was exposed by denudation. It was probably at the 

 close of volcanic activity that a melaphyre dyke was formed 

 resembling the porphyritic olivine-basalt of the Lion's Haunch, 

 Edinburgh. 



At Poortown an intrusive mass occurs, provisionally termed 

 augite-picrite-porphyrite, and considered by Mr. J. G. Gumming 

 to be of post-Carboniferous age. 



Numerous dykes of ophitic olivine-dolerite occur between Bay-ny- 

 Carrickey and Castletown Bay, at Langness, &c. They are post- 

 Lower Carboniferous, and possibly of early Tertiary age. 



Full details with regard to the development and the macroscopic 

 and microscopic characters of the various igneous rocks are supplied 

 by the author, who acknowledges his indebtedness to Prof. Boyd 

 Dawkins for the use of his geological map and notes. 



April 22. — Dr. A. Geikie, F.E.S., President, in the Chair. 

 The following communications were read : — 



1. " Results of an Examination of the Crystalline Rocks of the 

 Lizard District." By Professor T. G. Bonney, D.Sc, LL.D., E.R.S., 

 Y.P.G.S., and Major-General C. A. McMahon, E.G.S, 



The authors, in company with the Rev. E. Hill, spent a consi- 

 derable part of last August in examining anew those sections in the 

 Lizard district which had any bearing upon the questions raised 

 since the publication of Professor Bonney 's second paper in 18813. 

 They had also the advantage of occasional conference with Mr. Teall 

 and Mr. Eox, whose valuable contributions to the knowledge of the 

 crystalline rocks of this district are well known. 



That the Lizard serpentines are altered peridolites may be regarded 

 as settled, but doubts have been expressed as to their relation to 

 other associated rocks, and as to the meaning of a streaky or banded 

 structure exhibited by certain varieties. 



The authors, after re-examination of a large number of sections, 

 feel no doubt of the accuracy of their original view that the peridotite 

 was intruded into the hornblende schists and banded " granulitic" 

 rocks, after these had assumed their present condition. In it they 

 find no signs of any marked pressure-metamorphism, either prior or 

 posterior to serpentinization. They have failed to connect the 

 streaky or banded structure with any foliation or possible pressure- 

 structure in the schists, and they can only explain it as a kind of 

 fluxion -structure, viz. as due to an imperfect blending of two magmas 

 of slightly different chemical composition, anterior to the crystal- 

 lization of the mass. 



The Porthalla sections have been examined with especial care, not 

 only because the serpentine is nowhere so conspicuously banded, but 

 also because its intrusive character has been denied, both it and the 

 hornblende schists being ascribed to the alteration of a series of 

 sedimentary rocks of suitable composition. Eor this view the 



