330 Proceedings of Royal Society of Edinburgh. [sess. 
Has 
Fig. 2. — Orthographic projection on b {010. } A simple crystal drawn so as 
to show the general habit of each individual of the Albite twins. The 
forms shown are the brachypinacoid { 010 } ; the unit prisms { 110 } ; 
and the brachyprisms { 130, 150 } ; the basal pinacoid { 001 } ; the 
negative macrodomes (101, 20l and 405), the latter being unusual ; 
and the right lower hemipyramid (111). 
Fig. 3. — Clinographic projection of a complex group of five individuals. 
Each is a polysynthetically-twinned aggregate of many sub-individuals, 
and the individual crystals are again combined in accordance with the 
usual Albite Law. The forms present are : — The brachypinacoid, 
b { 010 } ; the basal pinacoid, c { 001 } ; the right .unit hemiprism, 
m (110) ; and its left analogue, M (110) ; with the left brachvprism, 
2 (130) ; the negative macrodomes, x (101), y (201), and E (203) ; 
together with a negative hemipyramid of the unit series, which 
appears to be A {443}. 
Forsterite. — When Mr Clough of the Geological Survey was 
examining the metamorphic marbles of the Glenelg district he dis- 
covered several minerals of considerable interest. To these was 
afterwards added the mineral named above, which was identified 
by Messrs Teall and Pollard, and was described by them in the 
Quarterly Journal of the Geological Society of London, vol. Iv. 
p. 372. The marble had been considerably altered by both 
dynamic causes and by hydrothermal action ; so that the impurities 
originally present in it, together, possibly, with other constituents 
carried into the rock by the heated waters, eventually combined 
