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University of California. 



[Vol. 2. 



a facies of the complex peridotite area at Point Morrito. Here is a 

 small body of rock, a segregation from the magnesian magma which 

 surrounds it on all sides. It consists of feldspar and hypersthene 

 in varying proportions, and, occasionally, small grains of magnetite. 

 The constituents are fresh and free from interpositions. With 

 decrease of feldspar there are transitions to a rock consisting almost 

 wholly of hypersthene, and with the addition of augite and olivine 

 gradations to the more common peridotite. A portion of this norite 

 is very coarsely crystallized, the crystals reaching a diameter of one 

 and a half inches. 



The typical olivine-gabbro is an abundant rock, and is represented 

 in the collection by many slides. It often contains a little hyper- 

 sthene and shreds of brown hornblende. The olivine occurs in 

 irregular-shaped aggregates, and in those rocks which are banded 

 it is strung out in more or less connected patches. The beginning 

 of alteration is usually apparent in a yellowish border and an inter- 

 secting network of cracks. Although the first mineral to separate 

 out, it is never idiomorphic. A number of inclusions of olivine in 

 augite were observed where the grains of the former had a very 

 perfect elliptical outline. One of the most striking phenomena 

 associated with these olivine-bearing rocks is the system of radiating 

 cracks which extend out into the feldspar from the olivine aggre- 

 gates. These cracks are grouped in parallel or radial bunches, 

 diverging from the projecting points of the olivine aggregate and 

 extending through the intervening feldspar, to the adjoining aggre- 

 gates of the same mineral. The extent to which this is developed, 

 even in those rocks in which alteration is but slightly apparent in 

 the olivine, is very remarkable. 



In the coarser rocks, and in the larger individuals of augite in 

 most specimens, there is observed the beginning of alteration indi- 

 cated by the development of an assemblage of fine parallel cleavage 

 cracks and polarizing interpositions. In the banded varieties, the 

 olivine is segregated in layers; other layers consist of olivine and 

 augite, between which are the feldspathic bands, containing some 

 augite, but free from olivine. The fresh condition of the augite, 

 which in many sections has associated with it partial rims or shreds 

 of brown hornblende, make it fairly certain that the latter is original, 

 and was the last constituent to crystallize out. 



