from the Standpoint of Physical Chemistry. 225 



individuals of different extinction. Single units show dimen- 

 sions of 0*2 by 0*lo mm with occasional larger grains up to - 5 ram 

 in size. The crystals are almost colorless, with a faint tinge of 

 brown or green. In many slides twinning is a common char- 

 acteristic. Twinning and composition plane the orthopinacoid 

 (100). Polysynthetic basal twinning is absent. Optical 

 character biaxial and positive. No difference in dispersion 

 between red and violet can be perceived. Prismatic cleavage 

 often poor in the small grains but well developed in the larger. 

 Birefringence 0'020-0'022. 



Extinction measured from the ortho-pinacoidal twinning 

 plane to the axis of minimum elasticity, Z, gave angles of 45 

 degrees. The mineral is evidently not pure diopside, but 

 whether it varies toward hedenbergite or toward augite would 

 be difficult to determine without chemical analysis. It is 

 referred to as diopside. The quantity is somewhat in excess 

 of the plagioclase. 



The magnetite is seldom in euhedral crystals, more often 

 in fine dust or in trellis-like or fir-tree groups. During the 

 crystallization of the plagioclase and diopside the magnetite 

 appears to have been mechanically pushed aside until the last 

 stages of crystallization were reached, and then included within 

 the final crystals of the lighter constituents, to which it gives a 

 dark brown color. Even in the final stages the plagioclases 

 seem to have had the power to free themselves of the magnetite 

 dust fairly effectually, and most of it is included within diop- 

 side grains. A small portion is enclosed within irregular 

 patches of light-green chloritic material, which may represent 

 resorbed olivines subsequently chloritized. 



In addition to these essential constituents there are a number 

 of well-defined areas of what was originally olivine, but which 

 show strong resorption and later alteration to serpentine and 

 chlorite. 



The magnetite in these rocks is probably titaniferous, as it 

 gives rise on decomposition to milky-white leucoxene. 



In examining those slides which" have the texture normal to 

 basalts the plagioclases appear to have the crystals developed 

 in more nearly euhedral forms than the diopside, and it might 

 be argued that this is confirmatory of the generally accepted 

 view that the growth of the plagioclases was finished before that 

 of the diopside began, and that the latter mineral molded itself 

 around the plagioclase. It is only necessary, however, to 

 devote a little study to the question to determine that this is 

 emphatically not the case. In those vitrophyric rocks first 

 described (e. g. No. 105 and No. 57) in which plagioclase and 

 diopside undoubtedly developed while swimming freely in a 



