I06 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



of the crater may reach. In no case can an analysis of ash give 

 an accurate idea of the composition of the magma. 



The structure and composition of igneous rocks were next dis- 

 cussed. It was shown that such minerals as amphibole, orthoclase, 

 and some micas are pre-eruptive as to crystallization, whereas leu- 

 cite and its allies, with pyroxene, are either formed in dykes under 

 low pressure or by prolonged recuit in the volcanic chimney, or after 

 expulsion. The order of formation of these minerals is quite diffe- 

 rent from their fusibilities, because they may be regarded as dissolved 

 in the magma, and separate according to their degree of solubilit}' 

 in the glassy magma and according to the different elements, their 

 proportions, and their affinities. The part played by the chlorides 

 and sulphates as solvents of silicates was discussed, and it was shown 

 how by their decomposition the hydrochloric and sulphuric acids 

 escape in the volcanic vapour, and how the bases may aid in render- 

 ing the magma more basic. These facts are borne out in the arti- 

 ficial production of rocks and minerals. 



The author maintained that throughout the whole paper his 

 arguments were based on known facts and physical laws. 



Discussion. 



Prof. Peesiwich was inclined to agree with the author in his 

 .view that the contact of heated lava with water-bearing strata was 

 an efficient cause of volcanic activity, but on some other points he 

 could not agree with him. 



The following specimens were exhibited : — 



Specimens of Echinoderms showing the structure of the ambulacra, 

 exhibited by Prof. P. Martin Duncan in illustration of his paper. 



May 13, 1885. 

 Prof. T. G. Bonnet, D.Sc, LL.D., P.B-.S., President, in the Chair. 



WiUiam Horton Ellis, Esq., J.P., Hartwell House, Pennsylvania, 

 Exeter, and Prof. J. Hoyes Panton, M.A., Agricultural College, 

 Guelph, Ontario, were elected PeUows ; and Prof. J. Gosselet, of 

 Lille, a Eoreign Member of the Society. 



The List of Donations to the Library was read. 



The Secretary announced that Dr. John Evans, F.E.S., E.G.S., 

 had presented a copy of his photographic portrait. 



The following communications were read : — 



1. "On the Ostracoda of the Purbeck Formation; with l^otes 

 on the Wealden Species." By Prof. T. Rupert Jones, F.R.S., 



F.a.s. 



