80 



chrome-diopsides of various localities, although be does not appear to have 

 detected chromium in this case. Possibly it was not sought for. The 

 microscopic appearance of the mineral in question is represented in Fig. 1, 

 Plate V. 



]o<Ki7 98-694 99-37 100-67 99-833 



T. Chrome-diopside, from the Lake of Lherz, (DAMOUE). W 



IT. Do. Sclnvar/e Steine. DiHgegend, (OBBBBKK). (?) 



III. Do. Olivine-nodule. Kosakomer, (FARSKY).^' 



IV. Do. Lherzolite. Piedmont, (Co88 A). <* 



V. Diopside. Halival, (HKDDLE).< 5 > 



In the rocks of the picrite group we have several varieties of monoclinic 

 pyroxene. These varieties are distinguished from each other under the 

 microscope by colour, pleochroism, cleavages, and the extent to which 

 crystalline form is developed. Some are perfectly colourless, others a pale 

 yellow or brown, others brown, reddish-brown or violet. In some cases a 

 change f roih a nearly colourless to a deeply coloured variety may be observed 

 in one and the same individual. 



In several varieties of the hornblende-picrites described by Professor 

 BONXEY a colourless mineral occurs, sometimes in the form of irregular grains 

 and sometimes with perfectly definite crystalline form. In the case of a 

 boulder from St. David's ((i) the crystals show the brachypinacoids (010), 

 the orthopiuacoids (100), the faces of the prism (110), and the positive 

 hemi- pyramid (111). The prismatic faces arc less pronounced than the 

 pinacoids in the zone of the vertical axis, and the terminal faces are some- 

 times ill-developed. Transverse sections show the usual prismatic cleavages, 

 and give an optic axis in convergent light. Longitudinal sections with 



(1) B.S.G., 2nd ser., XIX., p. 413. 



(2) Ein Beitrag zur Kenntniss des Palaeopikits, Inaug. Biss. Wlirzburg. 1877. 



(3) Verb, d.k.k. geol. Reichs. Wien , 1876, IX., p. 207. 



(4) Quoted from Roth, Beitrage zur Petrographie. &c. Abh. d.k. preus. Akad. Wiss. 

 Berlin, 1884, p. 13. 



(o) Trans. Roy. Soc. Edin., vol. XXVIII. (1879), p. 479. 

 (6) Q.J.G.S., vol. XLL, p. 519. 



