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University of California. 



[Vol. r. 



already mentioned, where the invading mass was too insignificant 

 to bring about much alteration in the surrounding strata. It will 

 be seen, on turning to the map, that there are numerous and con- 

 siderable breaks in the continuity of the metamorphosed zones. In 

 the greater number of cases this merely indicates that no schist out- 

 crops from the soil, although it may be present beneath it. In one 

 or two instances, however, in which the contact between the four- 

 chite and the sandstone has been exposed, the alteration has not 

 gone far enough to obliterate the ordinary clastic structure of the 

 latter, and appears to be confined to a crushing, darkening* in color, 

 and extensive veining with calcite or quartz. Only such portions 

 have been mapped as metamorphic rock as have actually been more 

 or less recrystallized or in which there is good reason to suppose 

 such action has taken place. It will be apparent from the foregoing 

 that the island does not afford good opportunity for studying the 

 various steps of alteration at varying distances from the igneous 

 mass. It is possible, however, to demonstrate its existence and to 

 indicate its character. 



The most interesting mass of schist exposed upon the margins 

 of the large fourchite sill is indicated upon the map, on the upper 

 side of the eastern limb of the crescent-like exposure. A single 

 mass, of perhaps five feet in diameter, here shows three facies of 

 very different appearance, according as different minerals predomi- 

 nate in each. Portions are often brilliantly white, with a granular, 

 saccharoidal texture, evidently made up chiefly of feldspar, in which 

 are disseminated a few scales of brown mica and needles of blue 

 amphibole. In other portions the atnphibole is much more abun- 

 dant, giving the rock a bright blue color, and still others are dark 

 brown from the preponderance of the brown mica. The different 

 colored bands are often quite sharply separated from each other. 

 A portion of the rock, showing all three minerals mentioned, was 

 sliced and placed under the microscope. It shows large crystals of 

 blue amphibole, generally without regular crystallographic termi- 

 nations, and usually clustered into aggregates, lying in a clear, color- 

 less matrix. 



Cross sections of the amphibole show, besides the dominant 

 prismatic faces, a slight development of the macropinacoidal and 



