PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 



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Gabbro.] 



peculiar included plates, coincident with cleavage, which gives them a specked or 

 spotted aspect. Another peculiarity of the augite in this rock is its including poikiliti- 

 cally a number of smaller augites which have different orientation. This is shown in 

 the upper portion of the figure below. 



The olivine in this rock is abundant, and, like the augite, is quite light colored, 

 indicating its low content of iron, and concordant with its lateness in date. It uni- 

 formly embraces the feldspars ophitically, but is embraced by the augites. This is 

 not a usual structure for olivine. One of the larger grains is represented below. In 

 this figure those areas marked a are of olivine and have simultaneous extinction in 

 the direction shown by the arrow. Those marked b are of augite and have extinc- 

 tion in various positions, while those marked c are of labradorite-bytownite. This 

 was also photographed. (See plate I, figure 7.) 



FIG. 18. OPHITIG STRUCTURE OF OLIVINE. 



In the upper part of the figure, where four orientations of augite are shown, the 

 mineral contains numerous inclusions. It appears as if the crystallizing force in the 

 augite, the last mineral to be generated from the magma, had embraced the remnants 

 of the glass and undifferentiated magma. In the small isolated figure at the top of 

 the figure a distinctly cleaved augite, evidently corroded, is surrounded by a later 

 augite having the same orientation, the whole exactly filling an angular interstice 

 between feldspars. In other cases two augites, similarly situated, have different 

 orientations. 



One section. 



Age. Cabotian. 



Remark. This is a beautiful rock in thin section, owing to the good preserva- 

 tion of the elements, even the olivine being nearly intact. The section examined by 

 Wadsworth is very thick, and the feldspars appear fissured and darkened. N. H. w. 



