Colloid Silica in the Loicev Clialk of Berkshire. 433 



are so well known in other territories of old crystalline rocks — 

 Saxony, Brittany, Scandinavia, Scotland, the Hudson River, etc. — 

 constitute in Madagascar, as they do at KUima-njaro on the 

 adjacent mainland, a large part of the ancient platform on the sub- 

 merged portions of which the sedimentary rocks have accumulated, 

 and through which the volcanic lavas were erupted. 



ii. The Volcanic Bods. — In composition these are acid, inter- 

 mediate and basic, mainly the latter. The acid and intermediate 

 types described are sanidine-ti-achi/te and hornblende-augite-andesite. 

 The basic rocks consist of various types of basalt. They vary with 

 respect to the presence or absence of corroded quartz-grains, 

 olivine, porphyritic hornblende, and biotite. In one interesting typo 

 the hornblende appears in small idiomorphic crystals as a constituent 

 of the ground-mass. A felspar-free variety, or magma-basalt, is also 

 represented. This rock contains only a small quantity of olivine, 

 and is therefore intermediate between Eosenbusch's Limburgite and 

 Bolter's atigitite. 



March 20.— W. T. Blauford, LL.D., F.E.S., President, 

 in the Chair. 



The following communications were read : — 



1. " Supplementary Note to a Paper on the Rocks of the Atlantic 

 Coast of Canada." By Sir J. W. Dawson, K.C.M.G., F.R.S., F.G.b. 



In a paper in the Quarterly Journal of the Geological Society for 

 November 1888, the author referred to the Olenellus-fsiuna. as cha- 

 racterizing the Middle Cambrian. This fauna, he has now no doubt, 

 from the recently published observations of Walcott and Matthew, 

 should be regarded as characteristic of the Upper Member of the 

 Lower Cambrian. From this arises a new view of the physical 

 geography of the period, namely, that the Lower Cambrian was, 

 in America, a period of continental depression, and the Middle 

 Cambrian a period of continental elevation, leading to the important 

 conclusion that a time of elevation intervened between the Huronian 

 and the early Cambrian, which may I'epresent the apparent gap 

 between these systems in Eastern America. He thinks that this 

 new view deserves a special mention in connexion with the proba- 

 bility that the Huronian and Kewenian beds are of littoral origin. 



2. " The Occixrrence of Colloid Silica in the Lower Chalk of 

 Berkshire and Wiltshire." By W. HiU, Esq., F.G.S., and A. J. 

 Jukes-Browne, Esq., F.G.S. 



In the Lower Chalk of Berks and Wilts are beds which contain a 

 large amount of disseminated colloid silica ; these are comparable in 

 general structure to the Malmstones of the Upper Greensand, Dr. 

 Hinde's study of the latter led him to believe that the globular 

 colloid silica which they contain was directly derived from the 

 remains of siliceous sponges, and the authors' studies of the Chalk 

 specimens have confirmed this conclusion by adding several important 

 pieces of evidence. 



Fhil. Mag. S. 5. Vol. 27. No. 168. May 1889. 2 F 



