PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 529 



Porphyryte.] 



the groundmass. Other sections of the less altered rock show that the groundmass 

 is composed of interlocking laths of feldspar, sometimes the feldspar has a tendency 

 towards a granular development, as stated below. 



Macroscopically the porphyryte is seen to have an aphanitic groundmass, which 

 varies in color from a reddish purple to a dull olive green; the freshest and more 

 abundant phases show the purple color. In this groundmass are sharply outlined 

 shining black crystals of hornblende and also irregular greenish areas, in and around 

 which are frequently small bright yellow spots and small white spots. Under the 

 microscope the grouudmass is seen to be made up entirely of small, short interlacing 

 laths of feldspar. In some sections the groundmass becomes coarser and there is a 

 decided tendency towards a granular development of the feldspar. Although 

 polysynthetic twinning is common, still some of the feldspar does not show it and 

 seems to be monoclinic in character; that there is considerable orthoclase in the 

 rock is also indicated by the percentage of potash in the analysis.* The feldspar is 

 undergoing alteration and is filled with small inclusions and minute fibres which 

 sometimes appear to be sericite. The rock is crowded with dust-like particles, to 

 which, perhaps, is due its purple color. There seems to have been but one period of 

 crystallization for the feldspar, phenocrysts of this mineral being entirely absent. 

 In all the slides examined there is no unindividualized glassy matter to be seen. 



The hornblende is of the usual brown basaltic variety, but the pleochroism is 

 not as intense as in most basaltic hornblendes; a is light straw colored, b is yellowish 

 brown, and c is olive brown. The rays vibrating parallel to b and c are not very 

 unlike. The absorption formula is c ba or c>bi. The hornblende is all 

 porphyritic in character, the individual crystals being from one to five millimeters 

 in length. Each phenocryst is usually surrounded by a dark corrosion rim. 



The dull greenish areas seen in hand specimens are found to be aggregates of 

 rlilorite scales, sometimes with a radial arrangement. It is evident that some of this 

 chlorite is an alteration product of the hornblende. But most of the chlorite areas 

 give no evidence as to their origin. In one section a core of pyroxene was seen in a 

 chlorite area, and it is possible that many of these chlorite areas represent old augite 

 phenocrysts; however, they do not show the characteristic outlines of augite crystals, 

 but are usually irregular. Even if all of these chlorite areas, which are not clearly 

 alteration products of hornblende crystals, represent original augite individuals, the 

 hornblende would still be in excess of the augite. So the rock is called a hornblende 

 porphyryte, although it perhaps originally was an augite hornblende porphyryte. 



The yellow spots in and around the chlorite areas are secondary epidote, often 

 in the form of minute spheres. And small white spots of calcite are also seen. 



*Dr. Whitman Cross, who kindly examined slides of this rock for the writer, also thinks that considerable orthoclase is 

 present. 



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