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ANNALS NEW YORE ACADEMY OF SCIENCES 



Petrographic examination of the unaltered basalt in the area with 

 which we are more especially concerned accords with the above analyses 

 and shows a rock of simple mineralogic composition. The components 

 are a pyroxene approaching diopside in optic properties, but shown by a 

 large extinction-angle to deviate somewhat from the typical mineral ; a 

 plagioclase feldspar of the composition of labradorite ; and magnetite. 

 The texture is normal ophitic. The appearance is illustrated in Plate XI,. 

 fig. 1. . ■ 



In both the crystalline and glassy facies, remnants of phenocrysts of 

 resorbed olivine are not uncommon. These phenocrysts appear to have 

 been developed during a period of intra-telluric crystallization and to 

 have become unstable as a consequence of changes in temperature and 



Fig. 2. Chloritic nodules from resorption of olivine (X: 



Slide 48. 



open field). X 35. 



pressure attendant upon extrusion, with simultaneous variation in the 

 composition of the magma consequent upon the evolution of contained 

 vapors. Eesorption of olivine by the magma often proceeded so far that 

 the only traces of the mineral that survive are small, spheroidal nodules, 

 which, by subsequent alteration, have been converted almost whollv 

 into chlorite or serpentine (fig. 2). The general character of the 

 crystallized rock and such special features as the resorption of olivine are 

 of importance in following out the process of formation of the secondary 

 minerals; for vestiges of the primary character often remain even in ad- 

 vanced stages of alteration and show clearly that much of the action of 

 recrystallization proceeded from stage to stage in a close-textured rock, 

 without the presence of any important cavities. 



A study of the field relations has shown that the spheroidal masses of 



