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University of California . 



[Vol. 3. 



blende clearly predominates. The grain is line, the hornblende 

 averaging .31 mm in size of crystals, and the feldspar .25 mm. 

 Very few crystal faces are developed, the rock presenting a good 

 hypidiomorphic granular structure. That which in the hand 

 specimen looks like flow structure is composed of rough lines of 

 feldspar crystals somewhat larger than the average. The horn- 

 blende crystals often have a rough approximation to an align- 

 ment parallel to these feldspar bands. This would indicate that 

 the rock was subjected to pressure when in a more or less plastic 

 condition. The roughly lens- shaped masses of the rock would 

 seem to indicate the same. In some portions of these bodies the 

 rock has been fractured in parallel lines, which have become 

 filled with secondary epidote. 



In those portions of the rock which show no traces of parallel 

 arrangement of the crystals, no decomposition products occur; 

 in that which shows effect of pressure a little hornblende has 

 changed to epidote and the feldspars are clouded. 



The hornblende has the same characteristics as that of the 

 main mass. The two varieties, light and dark colored, appear in 

 the same manner. The differences are: first, a much greater 

 tendency to twinning on (100) ; second, the parting on (T01) is 

 shown only in a few of the larger crystals; and lastly, no traces 

 of a parting on (100) can be found. 



The feldspars occur in crystals elongated parallel to the 

 albite twinning lamellae. Only traces of cleavage are present. 

 Zonal structure is found, but not well developed. The range in 

 composition of these plagioclases is from andesine. with a maxi- 

 mum extinction angle of 20°, to acid labradorite, with a 

 maximum angle of 35°. They thus correspond with the feld- 

 spar of the main mass, but are a trifle more acid. All characters 

 are the same. The magnetite is in small grains in size up to 

 .1 mm, scattered throughout. It often shows well developed 

 octahedral form. In this rock phase it also shows undoubted 

 cleavage parallel to the octahedral faces. In some of the larger 

 grains the grinding of the rock slide has caused the appearance 

 of a series of parallel cleavage faces, which show high metallic 

 luster. The biotite and apatite occur in relatively small amount 

 and do not differ from that already commented on. 



