40 SELECTED NOTES FROM 



Orthoclase is often found in twins (Carlsbad twins), but 

 never twinned like Plagioclase. 



J. M. Mello. 



Selectcb 1Rote0 from tbe Socicti^'e 



INORGANIC. 



Elvanite (PI. 2, Figs, i, 2), or Quartz Porphyry, is a rock 

 which occurs in veins piercing through the granite, and is therefore 

 a more recent formation. It will be seen in the Microscope, 

 especially when the Polariscope is used, to consist of a matrix of 

 dull-looking orthoclase Felspar, sometimes forming twins when 

 definite crystals are developed in it. The matrix will be found to 

 enclose numerous clear grains : these are grains of quartz por- 

 phyrytically developed. They will display fine colours when 

 polarised. Some accessory minerals occur in small quantities, — 

 amongst them a green one, which is pleochroic ; its behaviour is 

 best observed by removing the analyser and rotating the polar- 

 iser. It is probably Chlorite, a secondary product in the rock, 

 indicating a slow process of change. Oligoclase and Sanidine 

 sometimes are present in Elvanite as well as Orthoclase. 



Crysolite, or Olivine (PI. 5., Fig. 3) (from Atraja, in the 

 Sandwich Isles), is apparently in the form of water-rolled grains, 

 although it must have been originally derived from one of the 

 neighbouring volcanoes. Very few of the grains show very 

 distinctly the peculiar appearance by which Olivine may be so 

 readily recognised in microscopical sections of Olivine-bearing 

 rocks — viz., an uneven-looking, granular surface, usually present- 

 ing in the polariscope bright rosy and green colours. 



Diorite is a rock consisting essentially of Plagioclastic 

 Felspar and Hornblende; the Plagioclase may be seen, espe- 

 cially when the Polariscope is used, to form a network of 

 beautiful Crystals, showing more or less of twin compositions, 

 the Crystals forming the groups exhibiting striae of various colours. 

 The Hornblende is best recognised by moving the analyser; 

 when, on the rotation of the polariser, the characteristic dichroism 

 of this mineral will be seen. 



J. M. Mello, 



