360 W. B. Emery— Igneous Geology of Carrizo Mountain. 



Macroscopic description. — In the hand specimen numerous 

 phenocrysts of a rather pinkish feldspar, quartz, and horn- 

 blende are seen set in a light grayish aphanitic groundmass. 

 An occasional phenocryst of iron ore is present. 



Microscopic description. — Under the microscope the pheno- 

 erysts megascopically observed are seen set in a minutely 

 granular gronndmass of quartz and feldspar. 



Hornblende is idioinorphically developed in long, slender 

 prisms. It is of a somewhat lighter green color than common 

 green hornblende and in places has a distinctly brownish tinge. 

 In it the iron ore is abundantly concentrated. The horn- 

 blende is largely altered to chlorite. ' 



Feldspar occurs in two generations. As phenocryst it is 

 present in a few large crystals which show both Carlsbad and 

 albite twinning and have been determined to be andesine with 

 a chemical composition of about Ab 06 An a5 . The feldspar of the 

 groundmass shows albite twinning in places and for many 

 crystals the index of refraction is higher than that of Canada 

 balsam, indicating its plagioclase character. As the feldspar is 

 developed in long, tabular crystals, many of the pieces seen in 

 thin section have a square outline, and as extinction takes 

 place parallel or very nearly parallel to the side this feldspar 

 is thought to be an acid andesine. 



Quartz is present in two, and very probably in three genera- 

 tions. It occurs as phenocrysts, in the groundmass, and very 

 likely combined with feldspar in the interstitial glass. As 

 phenocryst the quartz is seen in large rounded forms which 

 exhibit a nearly square cross-fracturing. About these masses 

 the groundmass is much finer grained than elsewhere in the 

 slide and it is supposed that the quartz, partially crystallized 

 before, has been resorbed, thus giving rise to the rounded and 

 embayed outline and a zone of more siliceous character sur- 

 rounding it. 



In the groundmass the quartz is seen to have square outlines 

 and to extinguish diagonally with respect to the side of the 

 square. Quartz of this character, in minute clihexahedrons, 

 has been described by Kiich as being very characteristic of 

 certain dacites of the Andes,* and it is interesting to record 

 that in Carrizo Mountain there is found another example of 

 this somewhat rare mode of occurrence of this mineral. 



The texture of this dacite is porphyritic. The groundmass 

 is of microgranular character with a small amount of glass, 

 which very probably consists of uncrystallized quartz and 

 feldspar, filling up the minute interspaces. 



* Kiich, Richard, Geologische Studien in der Republic Colombia, I, Petro- 

 graphie, 1, Die vulcanischen Gesteine, 1892-93, pp. 68-69. 



