136 Reports & Proceedings—Mineralogical Society. 
The soundings made almost every day led to the discovery of 
an unexpected shallow area in the south-west. The continental 
shelf here has a breadth of at least 100 miles, and was followed 
over a distance of 300 miles. The depths clustered either about 
180-190 fathoms or 250-260 fathoms; the back and forward drift of 
the Endurance showed that the structure of the sea bottom there 
was a series of stepped plateaus. The abnormal depth of the 
continental shelf round Antarctica is probably due to earth move- 
ments. The position of the isobaths for 1,000, 1,500, and 
2,000 fathoms, and the drift of the ice practically settled the vexed 
question of “ Morrell’s land”, whose existence must now be 
considered highly improbable. 
MINERALOGICAL SOCIETY. 
January 18.—Mr. A. Hutchinson, Vice-President, in the Chair. 
A. F. Hallimond: The Olivine Group. 
Since the discussion of the densities by Thaddeef in 1896, and of 
the optical constants by Backlund in 1909, numerous additions 
have been made to the published data. These have been collected, 
and the most probable values for the pure compounds obtained. 
The molecular volume of monticellite is slightly greater than the 
mean between forsterite and y-Ca,SiO,. For the complex mixtures 
the density and mean refractive index yield additive relationships, 
but the birefringence and axial ratios follow no additive law. The 
conditions of plotting which must be observed if the variation of 
an additive property with composition is to be expressed by a 
straight line were briefly summarized. 
W. A. Richardson: A Method of Rock-analysis Diagrams based 
on Statistics. 
Oxide variation diagrams, similar to those employed by 
Dr. Harker, can be used for expressing the chemical relations of 
rock groups and individuals. The diagrams obtained from plotting 
Iddings’ selected analyses gave the maximum variation for all 
rocks. 
L. J. Spencer : Identity of Trechmann’s “ A-tin”’ with stannous 
sulphide. 
A re-examination of the original material described by C. O. 
Trechmann in 1879 as an orthorhombic modification of tin proved 
that he made his crystallographic determinations on crystals of 
one kind (viz. stannous sulphide), whilst the chemical analysis was 
made on crystals of another kind (viz. metallic tin). Tin, is there- 
fore dimorphous and not trimorphous, ““ white tin ”’ being tetragonal 
and “ grey tin’ cubic. Orthorhombic crystals of stannous sulphide 
(SnS) and tetragonal crystals of iron stannide (FeSng) from tin 
furnaces, and rhombohedral crystals of tin arsenide (SngAsq) 
isolated from a tin-arsenic alloy were also described. 
