328 Geological Society: — 



a. Gabbros or Dolerites, in which the originally constituent 

 minerals are either to a great extent unchanged, or, sometimes, 

 almost entirely represented by pseudomorphic forms. 



b. Killas, or ordinary clay-slates. 



c. Highly basic hornblendic rocks, exhibiting a tendency to break 

 into thin plates ; these under the microscope present the appearance 

 of metamorphosed slates. 



Slaty rocks of a character intermediate between b and c also occur. 



In the Cape-Cornwall district the " greenstones " are chiefly 

 hornblendic slates, sometimes with veins or bands of garnet, mag- 

 netite, or axinite. The rocks near the Gurnard's Head are almost 

 identical with those of Mount's Bay. The crystalline pyroxenic 

 rocks and metamorphic slates of the St. -Ives district exactly re- 

 semble those of Penzance. The greenstones between St. Erth and 

 St. Stephen's are probably altered ash-beds or hardened hornblendic 

 slates ; unlike the hornblendic and augitic rocks of the other 

 districts, they do not occur in the immediate vicinity of granite ; but 

 elvan courses are always found near them. The percentage of 

 silica in the two scries of rocks is nearly constant: the hornblende 

 slates contain about 10 per cent, less silica than the crystalline 

 pyroxenic rocks ; and there is an excess of iron oxides to nearly the 

 same extent, their composition in other respects being very similar. 

 The Killas is an acidic rock of essentially different chemical com- 

 position. 



2. " On Columnar, Eissile, and Spheroidal Structure." By the 

 Rev. T. G. Bonney, M.A., E.G.S. 



Some of the above structures have comparatively recently been 

 discussed by Mr. Mallet and Professor J. Thomson. Both these 

 authors agree in attributing columnar structure to contraction due to 

 loss of heat while cooling, but differ in their explanation of cross 

 jointing and spheroidal structure. In this paper it is sought to 

 show that the principle proved by Mr. Mallet to be the explanation 

 of the columnar structure is capable of a wider application. After a 

 brief notice of some instances of columnar structure, the author 

 described cases of a fissile structure seen in certain igneous rocks 

 (especially in the Auvergne phonolites), closely resemVing true 

 cleavage, and often mistaken for it — also the tabula:- jointing of 

 rodks, a peculiar form of this where most of the segments are of a 

 flattened convexo-concave form, spheroidal structure, and cup-and- 

 ball structure. He showed by examples that Prof. Thomson's 

 explanation of spheroidal structure was inadequate, and gave reasons 

 for considering all these structures to be due to contraction. He 

 also discussed more particularly the cup-and-ball structure, giving 

 reasons for thinking that the spheroidal and the horizontal fissures 

 were often to some extent independent of each other. 



March 8, 1876.— Professor P. Martin Duncan, M.B., E.R.S., Presi- 

 dent, in the Chair. 



The following communications were read : — 



1. " On the influence of various substances in accelerating the 

 precipitation of Clay suspended in water." By Wm. Ramsay, Esq., 

 Principal Assistant in Glasgow University Laboratory. 



