PETROGRAPIIICAL NOTES. 3 



Detailed Description. 



1. A specimen from Ensenada, corner of Galvez and 

 Tenth street, is macroscopically a coarse-grained rock with 

 fresh feldspar and quartz. Hornblende is present in much 

 larger quantity than biotite, and on the whole there are 

 more ferromagnesian silicates than usually. 



Under the microscope it is seen to consist of a hyphidio- 

 morphic granular mixture of quartz, plagioclase, hornblende, 

 moDoclinic and rhombic pyroxene and biotite. The mono- 

 clinic pyroxene ftrms ill-defined, short and colorless prisms, 

 considerably decomposed and obscured by chloritic and 

 uralitic aggregates. More frequent than the monoclinic 

 pyroxene, is a rhombic form of the same mineral, probably 

 hypersthene, occurring in irregular grains or short prisms; 

 the pleochroismus is distinct ranging from reddish to light 

 green; it is frequently decomposed in the same way as the 

 monoclinic pyroxene. The primary hornblende is of a 

 brownish green color, has the usual absorption and pleo- 

 chroismus, and is often intimately connected with the 

 pyroxene, being grown together with it or even surround- 

 ing it. 



Besides this normal hornblende, there is some uralite 

 formed from the pyroxene and distinguished from the first 

 mentioned mineral by its bluish green color. A yellowish 

 brown biotite in foils, which show no bending or compres- 

 sion, is also an imj^ortant constituent of the rock. Magne- 

 tite, much apatite and zircone occur. Among the feldsjDars 

 the plagioclase certainly predominates. There is probably 

 some orthoclase, although this cannot be definitely stated. 



The plagioclase is more or less idiomorphic,with generally 

 short lathlike or square forms. Often twinned accord- 

 ing to both the albite and the pericline law; zonar structure 

 wath correspondingly differing extinction is not unfrequent. 

 This plagioclase is probably an andesine or an oligoclase. 



