THE IGNEOUS liCXJKS. 493 



tlio midst of even the most eompiict ivmpliihole, ;niil l)ro\vii ocher colors 

 portions of most of the feldspars. lu spite of the fact that this rock has 

 been so changed, its diabasic structure remains, the compact hornblende 

 occupA'iug the place of the original augite, while the altered ])lagioclase, the 

 fibrous hornblende, and most of the biotite and of the chlorite occupv the 

 place of the original feldspai's. 



Some of the rocks from the interior of the knob (Specimens 22168 

 and 22173) are dark-gray, rather dense-looking, and perfectly massive, 

 exhibiting very decided luster-mottling, and showing plainly in the hand 

 specimen the diabasic structure. Under the microscope the augite that 

 remains is found to be limited almost exclusively to the mottled areas, and to 

 be the mineral to whose presence the latter is due. The pyroxene is almost 

 colorless. It occurs as very raggedly irregular cores surrounded by light- 

 green hornblende. It is penetrated by what Avere originally plagioclase 

 laths but are now larg-elv an aggregate of small saussuinte crystals, 

 betAA-een which are here and there patches of feldspar. The green horn- 

 blende is very slightly pleoclu'oic. It borders the augite mottlings, and 

 elsewhere in the sections it occupies wedge-shaped areas, penetrated, like 

 the augite areas, by laths of plagioclase. Chlorite is also present in small 

 quantities, and it likewise is in ophitic wedges. The plagioclase has nearly 

 all disappeared into its decomposition products. Leucoxene and epidote are 

 the only other important minerals present. The latter is in small A^ellow 

 grains, often in the midst of chlorite, and the former in line pseudomorphs 

 after crystals of ilmenite, cores of which occasionally remain in the centers 

 of the leucoxene masses. 



Other varieties of the rock (Specimens 22159, 221G0, and 22169), 

 from well within the rock mass, resemble very closely the augitic jjhases 

 described above, excej^t that they appear a little more altered, and their 

 plagioclase is in places reddened. Under the microscope they are aggi'e- 

 gates of light-green hornblende and chlorite, often stained with ocher, in 

 a mass of altered jjlagioclase whose princijial decomposition products are 

 epidote, saussurite, and chlorite. The epidote is embedded in chlorite, 

 either as pale-j^ellow plates or as little grains, often with crystal outlines. 

 Occasionally a large crystal of andesine, or of some plagioclase whose 



