238 Reports & Proceedings—Minerulogical Society. 
fractions arranged themselves according to density. Then within 
the central ultrabasic stratum there was a subsidiary gravity 
stratification—due to the subsidence of olivine crystals, giving rise 
to the graded mass described above. If differentiation had occurred 
subsequent to the arrival of the sill in the position that it now 
occupies, the contact rocks should have the same composition as 
the average rock of the sill. his, however, is not the case, as the 
average rock has the composition of an almost ultrabasic theralite, 
entirely different from the teschenites of the contacts. Hence it is 
believed that, after forming contact-sheaths of theralite, and under- 
going gravity stratification subject to liquation, the intrusion activity 
was renewed, and the sill was moved on along bedding-planes into 
cold-rocks, leaving its contact-sheath behind adhering to the old | 
contacts, and establishing new contacts with its upper and lower 
teschenite-layers. Here crystallization began, and, by the subsidence 
of olivine, the subsidiary gravity stratification of the central ultra- 
basic layer was effected. ‘The extraordinary flow-banding shown by 
the contact rocks affords confirmation of the renewed movement thus 
postulated. . 
In conclusion, the sill is compared with five other teschenite- 
picrite sills in Scotland, those of Ardrossan, Saltcoats, Blackburn, 
Barnton, and Inchcolm. 
II].—MuvyeratocicaL Socrery. 
March 21, 1916.—W. Barlow, F.R.S., President, in the Chair, 
Dr. J. W. Evans: A new Microscope Accessory for use in the 
determination of the Refractive Indices of Minerals. The accessory— 
a diaphragm with narrow slit adjustable in width—when placed in 
the primary focus of the objective or any point conjugate with it, 
serves several useful purposes. If placed parallel to the boundary 
between the two substances whose refractive indices are to be 
compared by the Becke method, it gives better results than an iris 
diaphragm. In the case of doubly refractive sections or grains in 
which an axis of optical symmetry lies at right angles to the microscope 
axis, the slit is placed parallel to the former axis, so that the paths of 
all the rays of light traversing it lie in a plane of optical symmetry 
and one direction of vibration is always parallel to the axis of optical 
symmetry, and a nicol is inserted so that the direction of vibration 
of the rays traversing it is parallel to the same axis: then the 
refractive indices of light vibrating parallel to that axis of optical 
symmetry may be investigated by the usual methods without the 
confusion caused by the bifocal images described by Sorby.—L. J. 
Spencer: A Butterfly Twin of Gypsum. In a well-developed twin- 
erystal, 6inches across, from Girgenti, Sicily, in which the twin- 
plane is d (101), the two individuals are situated on the same side of 
the twin-plane instead of on opposite sides as in the usual type.— 
Dr. W. R. Jones: The Alteration of Tourmaline. In a moist tropical 
climate minerals which are ordinarily regarded as stable break down 
to an extraordinary degree. At Gunong Bakau, Federated Malay 
