6 BULLETIN OF THE 



From what has now been said, it seems certain that a feldspar as basic 

 as labradurite exists in the rock. This is attested both by the high ex- 

 tinction angles in the zone of oP and oo Poo , as well as by the presence 

 of calcite as a significant product of alteration. That a feldspar less 

 basic than oligoclase, and probably as acid, is present also, is shown by 

 the zonal structure and wide range in specific gravity. The potassium 

 obtained from No. 222 may be derived either from alteration products 

 (uiuscovite 1) or from the feldspar itself. The analysis of this rock 

 m ioto, which may be found in the sequel, shows by the small amount 

 of hydration that little alteration has taken place. It therefore seems to 

 be certain that the potassium is derived from the feldspar itself, either 

 from orthoclase or from a potiish plagioclase. 



The principal non-feldspathic constituent is augite, which is much 

 broken up by feldspar crystals, though rarely the feldspar is penetrated 

 by augite, showing the nearly contemporaneous formation of the two 

 minerals. The augite, which has generally a rose color, displays a faint 

 dichroism, the ray parallel to b being pink, and that parallel to a pink- 

 ish yellow. Both prismatic cleavages are generally well developeil, and 

 intersect on the basal plane with the pyroxene angle 87°. Parting par- 

 allel to CO Poo and occasional twins according to the same plane, though 

 not constant features, are observed in the rock from some localities. 

 Zonal structure is not uncommon, aud rarely the hour-glass structure is 

 well developed. The principal alteration is to uralite, which is found in 

 rims completely surrounding many grains, while with others it has gone 

 farther, and found its way to the centre along the cleavage cracks. This 

 mineral occurs in its usual form in scales or sheaves, and is easily dis- 

 tinguished by its high double refraction, small extinction angle, and 

 strong pleochroism, the ray vibrating parallel to the long axis being 

 dark green, while that vibrating perpendicular to this direction is either 

 green or bright yellow. It is probable that the uralite further changes 

 to chlorite ; but since biotite was generally to be found in the vicinity 

 passing into chlorite, it could not be definitely determined. The distri- 

 bution of the calcite shows that it is derived from the pseudomorphism 

 of the augite, as well as from tlie decompositon of the feldspar. 



Compact green and brown (basaltic) hornblende, though noticed once 

 or twice, are extremely rare. 



The biotite when present is generally either in plates, from its pene- 

 tration of feldspar and augite, clearly original, or in fine scales or aggregate 

 masses in association with augite. The former variety contains inclu- 

 sions too small to be determined, with the characteristic pleochroic zones 



