Geological Society. 71 



diameter of either. This limitation is, of course, imposed by the 

 immense mathematical difficulties of the subject. But, within his 

 restricted limits, Mr. Thomson has done really good work, work 

 which enables us to have at least a glimpse beyond these limits. 



The latter portion of the essay, where chemical ideas of valency 

 &c. are introduced, is by no means so satisfactory as that which 

 precedes. Mr. Thomson is evidently far more at home among 

 symbols than among dyads and chemical molecules. 



X. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from vol. xvi. p. 456.] 



November 21, 1883.— J. W. Hulke, Esq., F.B.S., 

 President, in the Chair. 



npHE following communications were read : — 

 -*- 1. "On the Skull and Dentition of a Triassic Mammal (Tri- 

 tylodon longcevus, Ow.) from South Africa." By Prof. Owen, C.B., 

 F.E.S., F.G.S. 



2. " Cranial and Vertebral Characters of the Crocodilian genus 

 Plesiosuchus, Owen." By Prof. E. Owen, C.B., F.E.S., F.G.S. 



3. " On some Tracks of Terrestrial and Freshwater Animals." 

 By Prof. T. M c Kenny Hughes, M.A., F.GLS. 



December 5, 1883.— J. W. Hulke, Esq., F.E.S., President, 

 in the Chair. 



The following communications were read : — 



1. "On the Cambrian Conglomerates resting upon and in the 

 vicinity of some Pre-Cambrian Eocks (the so-called intrusive masses) 

 in Anglesey and Caernarvonshire." By Henry Hicks, M.D., F.G.S. 



The author stated that as he and others who had asserted thaj 

 some of the great rock-masses marked as " intrusive " on the maps 

 of the Geological Survey in xinglesey and Caernarvonshire were 

 really of Pre-Cambrian age, had been charged in the last edition of 

 the Geological Survey Memoir on North Wales with having done so 

 on purely theoretical grounds, he had found it necessary to obtain 

 the additional evidence which he now brought before the Society. 

 He felt this to be the more incumbent upon him as the present 

 Director-General had recently stated that his predecessor would not 

 admit the existence of any Pre-Cambrian rocks in the Principality, 

 and had further asserted that the author, " w T ith the most complete 

 disregard of the evidence by which the officers of the Survey were 

 led to regard certain rocks as intrusive " in Cambrian and Silurian 

 strata, had s'mply made these rocks into metamorphic and volcanic 



