BASIC INTKUSIVES. 201 



uralitic dolerites, epidiorites, or even diorites, as it is impossible, without a 

 sequence of changes, to determine whether tlie hornblende is original or 

 secondary. Rare rocks contain quartz in sufficient quantity to warrant 

 their designation as quartz-dolerites. However, they differ in no essential 

 respects from the other dolerites. The quartz is in micropegmatitic inter- 

 growth with feldspar, filling the angular interspaces of the rock. These 

 intergrowths were evidently the last elements to crystallize. 



The feldspar occurs in large automorphic lath-shaped crystals, which 

 in most cases show poly synthetic twinning. In a few cases unstriated crys- 

 tals were obsei'ved. Owing to the alteration of the feldspar, which has in 

 most cases almost completely destroyed the striations, it has been impossible 

 to make many accurate measurements. Measurements on the zone perpen- 

 dicular to 010 gave equal extinction angles against twinning planes of 37° 

 as maximum, showing the feldspar to be bytownite. The chief alteration 

 products of the feldspar are epidote and zoisite. With these are usually 

 associated more or less muscovite, some chlorite, and, more rarely, scales of 

 Ibiotite. Accompanying these alteration products one very frequently finds 

 limpid spots of secondary albite or quartz. Some few of the feldspars are 

 smoke-colored, and as the coloring appeared homogeneous even under the 

 highest powers, it would seem to be due to some pigment in the mineral 

 and not to minute inclusions. 



Pyroxene is very rare, having been observed in only a few sections and 

 in the majority of these is present merely as small remnants surrounded by 

 its secondary product, uralite. The pyroxene possesses the usual char- 

 acters of common augite. The augite is quite free from inclusions. Along 

 the edge its alteration to the light-green hornblende, uralite, can be readil}- 

 followed, and in one case an octagonal basal section of augite was observed 

 which was completely occupied by uralite fibers. 



The former presence of olivine is based upon very slight proof, viz, 

 the existence in some of the pyroxene and uralite crystals of areas which 

 are oval or round in shape and are occupied by joilite. The presence of this 

 pilite in the altered augite might possibly be explained as an alteration 

 product of the augite itself, but it is difficult to explain why pilite should 

 develop in one part of the augite and secondary coarse hornblende in the 

 other part. Moreover, the general characters of the rocks are such as to 

 lead one to expect to find olivine present in some of them. 



