Geological Society of London. 283 



2. " On the Igaeous Rocks of the South of the Isle of Man." By 

 Bernard Hobson, Esq., M.Sc, F.G.S. 



Omitting the Foxdale Granite, the oldest igneous rocks of the 

 island appear to be the diabase dykes of Langness, etc., intrusive ia 

 Lower- Silurian slates. The Crosby microgranite dyke is also 

 intrusive in these beds, and though its age is difficult to fix, 

 it is probably newer than the Foxdale Granite, which appears to be 

 of post-Lower Silurian and pre-Carboniferous age. 



Next come the volcanic rocks of lower-Carboniferous age — an 

 augite-porphyrite series consisting of tuff, breccia, agglomerate, 

 bedded lava, and intrusive masses exposed in a narrow strip 

 extending from Poolvash to Scarlet Point. A vent seems to have 

 been opened during or after the deposition of the Poolvash lime- 

 stone, from which fine volcanic ashes were ejected to form marine 

 tuff. At intervals between the eruptions the Poolvash marble was 

 deposited, and became interstratified with the tuff. The vent then 

 probably became plugged up, and a violent explosion following 

 supplied material for the agglomerate overlying the tufi". Lava then 

 welled forth, and finally the volcano became extinct, and the in- 

 trusive 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 porphy- 

 ritic 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. Cumming to 

 be of post-Carboniferous age. 



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

 Carrickey and Castletown Bay, at Langness, etc. 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. 



IL— April 22, 1891.— Dr. A. Geikie, F.R.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., F.R.S., 

 V.P.G.S., and Major-General C. A. McMahon, F.G.S. 



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

 siderable 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 1883. 

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

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

 crystalline rocks of the district are well known. 



That the Lizard serpentines are altered peridotites 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. 



