356 IT. B. Emery— Igneous Geology of Carrizo Mountain. 



The extinction angles are slight, S° from the cleavage lines, on 

 the face 010. Twinning parallel to 100 is common. Zonal 

 development is shown in a number of crystals and in some, 

 resorption. Euhedral outlines are well displayed. In some 

 places, the outline is completely preserved by the alteration 

 products ; in other places the outline is merely indicated by 

 the few remaining shreds of the unaltered mineral. Altera- 

 tion to chlorite and calcite, less commonly to epidote, takes 

 place. 



The plagioclase shows both Carlsbad and albite twinning and 

 was determined to be an andesine with a composition of about 

 Ab 4r An GS . Some crystals have a broad tabular outline, others 

 a lathlike development. The feldspar alters most abundantly 

 to kaolin, in less degree to calcite. Quartz when present as a 

 phenocryst is smaller than either the hornblende or the plagio- 

 clase. It is of the ordinary type. 



Iron ore, as mentioned, is present in places in two genera- 

 tions. As phenocrysts it is observed in places to have a square 

 outline indicating that it is magnetite ; elsewhere it is without 

 definite outline and is doubtless, in part, ilmenite. In not a 

 few places it was seen entirely enclosed by the hornblende, 

 indicating its earlier crystallization. It is abundantly concen- 

 trated about the borders of the hornblende both within and 

 just without the crystal. As a second generation mineral it is 

 scattered in small grains through the ground mass. 



Apatite is present in all sections studied, in some in such 

 large crystals and so abundantly as to almost merit being called 

 a phenocryst. It is of the ordinary type, in long prisms. 

 Titanite is of much less importance as an accessory than apa- 

 tite, and zircon is of less importance than titanite. Both 

 titanite and zircon are of the usual type. 



In the groundmass orthoclase is somewhat more abundant 

 than quartz. Both are developed in very small crystals. In 

 some places, however, the feldspar has a tendency to lathlike 

 development and there it is probably in part plagioclase. 



The texture of the rock as a whole is porphyritic. The 

 texture of the groundmass varies from microgranular, where 

 quartz and feldspar are present in about equal amounts, to 

 nearly trachytic, where feldspar is in excess of quartz. In 

 some slides there was observed a tendency toward a micro- 

 poikilitic development, and in one section a microspherulitic 

 texture was noted. 



Horiiblendic inclusions in the porphyry. — There are inclu- 

 sions of a hornblendic character present in the diorite porphyry. 

 These are ordinarily one or two inches in diameter, but were 

 observed to six inches in length. They are of hornblendic 

 nature so strongly, in places, as to suggest derivation from a 

 hornblende schist, and are in general of very angular outline. 



