140 Heports and Proceedings — Geological Society of London. 



and thrusting probably took place during the post-Carboniferous 

 period of earth- movement. Before the last movement fine-grained 

 diabasic rocks (' greenstones ') appear to have been intruded. The 

 thickening of the volcanic rocks to the southward seems to indicate 

 that the vents must have been situated in this quarter. The beds 

 as a whole were deposited in shallow water in the proximity of 

 land, and they point to the existence of rocks, such as granites, not 

 now known at the surface in the district. 



2. " A Process for the Mineral Analysis of Eocks." By W. J. 

 Sollas, D.Sc, LL.D., F.R.S., F.G.S., Professor of Geology in the 

 University of Oxford. 



The method proposed is to obtain a quantitative estimation of 

 the mineral composition of a rock, and from the known composition 

 of the minerals to calculate the percentage composition of the rock. 

 The specific gravities of the minerals are first determined by means 

 of a diffusion column of methylene-iodide and beads of known specific 

 gravity, and the presence or absence of particular minerals settled 

 for a certainty. Next, the separation of the minerals in a weighed 

 quantity of the powdered rock is undertaken by means of a special 

 separator ; the method being illustrated by the example of a rock 

 containing orthoclase (sp. gr. 2*56), quartz (2'65), andesine (2'67), 

 biotite (3'1), pyroxene (3'3), and magnetite. The first separation 

 would be with a liquid of sp. gr. 2'885, the mean of that of andesine 

 and biotite ; the next with a liquid of sp. gr. 2*66 ; the next 2'605, 

 and so on for the other constituents. The separated minerals are 

 dried and weighed, the loss distributed, and the analysis checked 

 by comparing the specific gravity of the rock in bulk with that 

 calculated from the specific gravity and proportion by weight 

 ot its constituents. In making choice of particular mineral analyses 

 for calculating the chemical composition, there are three guiding 

 principles : the analysis should be that of a mineral obtained from 

 the same kind of rock as the one under investigation ; if possible, 

 from the same locality ; and with the same specific gravity. The 

 process was tested on specimens of kentallenite supplied by Mr. Teall 

 and of gabbros from Skye by Mr. Barker, and in both cases the 

 results compared closely with those obtained from bulk analysis 

 of the same rocks. A further test was the comparison of the mineral 

 analysis by Miss Davies of a specimen of Devonshire granite with 

 Phillips's published analysis ; also of syenite from the Plauens'cher- 

 grund, and of tonalite from Adamello, with published analyses. 



II.— February 5th, 1902.— J. J. H. Teall, Esq., M.A., F.R.S., 



President, in the Chair. 

 Mr. H.. Bauerman, in exhibiting a remarkable Crystal of Cinnabar 

 from the Mercury-mines in the province of Kwei-chau (China), 

 observed that it was a completely developed penetration-twin of 

 two rhombohedra, attached to a mass of crystalline quartz. He 

 drew attention to the simple character of the form from this locality, 

 as compared with those of the crystals from Almaden and Avala. 



