142 Geological Society ; — 



* Electricity and Magnetism,' and a great number of articles, lec- 

 tures, and reviews ; the series of ' Encyc. Brit.' Physical articles, so 

 far as life permitted them to be written by Maxwell's pen, being 

 also inserted. It would have added to the interest if the dates had 

 in all cases been affixed to the different memoirs, but they seem 

 arranged in fairly chronological order. 



The mere titles of the papers carry the mind over a large part 

 of the field cultivated in recent years by what may be called 

 " Section A," and the world must be grateful to the Clerk Maxwell 

 Memorial Committee and the Syndics of the University Press for 

 the liberal manner in which the work has been carried out. 



O. J. Lodge. 



XIX. Proceedings of Learned Soci.eti.es. 



GEOLOGICAL SOCIETY. 



[Continued from p. 66.] 



December 17, 1890.— W. H. Hudleston, Esq., F.R.S., 

 Vice-President, in the Chair. 



nPHE following communications were read : — 



-*- 1. "On Nepheline Rocks in Brazil — II. The Tingua Mass." 



By 0. A. Derby, Esq., F.G.S. 



In a former paper the general distribution of the nepheline rocks, 

 so far as known, was given with a particular description of a single 

 one, the Serra de Pocos de Caldas. The present paper treats of a 

 second mass, the Serra de Tingua, a high peak of the Serra do Mar, 

 some forty miles from Rio de Janeiro. 



The peak is essentially a mass of foyaite rising. to an elevation of 

 1600 metres, on the crest and close to the extremity of a narrow 

 gneiss ridge of a very uniform elevation of about 800 metres. As 

 seen from a distance, the conical outline and a crater-like valley on 

 one side are very suggestive of volcanic topography. In the struc- 

 ture of the mass both massive and fragmental eruptives are found, 

 the former greatly predominating. 



The predominant rock is a coarse-grained foyaite which is found 

 everywhere in loose blocks about the margins of the mass, but not 

 extending beyond it. In the numerous cuttings in the immediate 

 vicinity, dykes of phonolite and basic eruptives (augitite) are ex- 

 ceedingly abundant, foyaite never appearing in a dyke form. There 

 is, however, abundant evidence that foyaite and phonolite are but 

 different phases of the same magma. 



Aside from the dyke phonolites, true effusive phonolites associated 

 with fragmental eruptives (tuffa) were found high up in the crater- 

 like valley, proving that the mass was a volcanic centre in the most 

 restricted sense of the word. 



This conclusion affords an explanation of some of the peculiarities 

 of the foyaite, which has many characteristics of effusive eruptivea 



