ULTRA BA81C INTEUSIVES. 213 



The rocks originally consisted largely of olivine, pyroxene, hornblende, 

 biotite, magnetite, and ihnenite. They now contain also, in considerable 

 quantity, a chloritic product which seems to have been derived from the 

 alteration of an original vitreous base. All of the specimens are exceed- 

 ingly altered. The original mineral constituents have to a great extent been 

 determined from their form, which in some cases has been preserved by the 

 products of alteration, and by certain structures in the pseudomorphs. The 

 minerals now composing the rock are original hornblende, biotite, apatite, 

 magnetite, and ihnenite, with secondary aniphibole, serpentine, chlorite, 

 calcite, sphene, and rutile. 



The two kinds of rocks, the gray and the dark-colored ones, were 

 evidently derived from rocks of essentially the same composition. They 

 have undergone different processes of alteration, and upon this depends the 

 difference in color. As the study of these picrites is chiefly one of the 

 alteration products of the minerals which composed them, it seems best 

 to describe separately the two rocks showing the different products of 

 alteration. 



GRAY TREMOLITIZED PICRITE-PORPHYRY. 



In the gray rocks the spots which are macroscopically observed are 

 found under the microscope to consist of an aggregate of minerals. Ijxami- 

 nation of these aggregates shows them to be entirely secondary. A careful 

 study of these aggregates shows them to consist of amphibole, magnetite, 

 ilraenite, and serpentine, the first being predominant. No trace of the origi- 

 nal minerals remains. The aggregates are the same in all of the crystals, 

 and the only clue to the original mineral is the form of the pseudomorphs 

 and certain structures in the aggregates. By means of the form the pheno- 

 crysts are readily divisible into three kinds. The first kind has a long 

 prismatic habit, with pyramidal faces meeting at rather an acute angle. 

 The iron oxide is arranged along certain lines, giving the characteristic 

 mesh structure of serpentinized olivine. The second kind is a short, thick 

 prism, for the most part with rounded ends, in some cases the pyramidal 

 faces meeting in a rather obtuse angle. The iron oxide in some of these 

 cases marks an imperfect parting perpendicular to the long direction of the 

 prism. These are supposed to be pseudomorphs after a pyroxene. The 

 third kind consists of round and irregular grains or plates, some of which 

 may be referred to pyroxene, others to olivine. 



