ULTRABASIO INTEITSIVES. 217 



The chlorite in the paramorphs after biotite shows extremely low blue 

 polarization color, and the characteristic pleochroism — yellowish, tinged 

 with red, when the rays vibrate perpendicular to the cleavage, and green 

 when 23arallel thereto. Apatite needles included in the biotite are inialtei-ed 

 in the secondary chlorite. 



. Some of the minute octahedral crystals in the amphibole pseudomorphs 

 after olivine appear to be slightly pellucid, with a brown color. If so, they 

 might be referred to picotite, but there is doiibt of the correctness of the 

 observation, in view of the high power used, the oil immersion lens, and 

 the fact that the search was for picotite. Close search was also made for 

 perovskite, but none could be found, unless the transparent crystals very 

 doubtfully referred to picotite are really perovskite. 



Forming the matrix in which the pseudomorphs after hornblende, 

 olivine, and pyroxene of these rocks lie, and frequently surrounding 

 isolated crystals, one sees an aggregate composed chiefly of a fine felt of 

 chlorite fibers. This alteration product contains a few apparently original 

 apatite needles, some secondarj^ grains of magnetite, and crystals of amphi- 

 bole which are colorless or else show but the faintest tinge of green, and are 

 larger than the amphibole crystals in the pseudomorphs. It is a secondary 

 amphibole very poor in iron, probably highly calcareous, and approaching 

 tremolite in composition. This chlorite aggregate shows no indication 

 whatever of crystal forms. It seems to be the product of a homogeneous 

 mass, such as would result from the decomi^osition of a vitreous base. Such 

 a base the aggregate is presumed to represent, although no trace whatever 

 of the glass has been observed in the i"0ck, nor in view of the altered con- 

 dition of the rock could such a glass be reasonably expected to still remain. 



DARK SBRPENTINIZED PICRITE-PORPHYRY. 



The second vai'iet}" of the picrite-porphyries is very dark greenish- 

 black, and represents the results of a slightly different jjrocess of alteration 

 from that by which the gray forms just described were produced. These 

 dark picrite-porphyries show a very much better developed porphyritic 

 structure than do the gray ones. This is due to the fact that the olivines in 

 these rocks w^ere well developed and reached a length of a centimeter. The 

 olivines are completely altered, serpentine, pilite, and magnetite, being the 

 products which form the pseudomorphs. The characteristic mesh structure 



