OF ARTS AND SCIENCES. 261 



and in polarized light, that the appearance to the unaided eye is cor- 

 roborated. We here find the cause in the fact that each spot is the 

 cross-fracture or cleavage of a cr}stal of pyroxene, which in crystal- 

 lizing has enclosed hundreds of feldspar crystals. 



The weathered surface is rusty gray, scarcely -^^ inch thick ; but it 

 is covered with knobs which are due to the more rapid destruction of 

 the materials between the pyroxene individuals. 



Examining thin sections under the microscope, we find the con- 

 stituents to \tQ plagioclase, pyroxene, olivine, and its alteration product, 

 as well as magnetite, and an unindividualized substance, both fresh and 

 altered, occupying interstices. 



In thin sections, the plagioclase is seen to exist in very sharply 

 defined and fresh, thin, tabular crystals, .001 to .002 inch thick, and 

 .01 inch, and less, long. It contains scattering interpositions of an 

 opaque black substance, and minute brown particles, which may be, 

 or have been, glass. 



The crystals of plagioclase have predetermined the contours of all 

 the other constituents, except the olivine, which crystallized first. 



The predominating feldspar is anorthite, as determined by the angle 

 between the principal sections in adjoining bands in the zone : it. 

 Scattering large crystals, which happen in the sections to be cut par- 

 allel to il., have their principal sections at an angle of 23° with the 

 edge : it, which would indicate albite or labradorite. 



The augite is very fresh and transparent, almost colorless in the 

 thin section, but with a tendency to purple-gray. An imperfect cleav- 

 age is indicated by somewhat irregular parallel fractures. It fills the 

 interstices between the closely packed individuals of feldspar in such a 

 manner that a single pyroxenic crystal encloses many hundreds of 

 these, while its crystalline integrity is shown by the uniform color in 

 polarized liglit, and by the arrangement of the cleavage cracks through- 

 out the area of the augite individual. 



It is a remarkable fact that, while these large individuals of pyroxene 

 contain thousands of feldspar crystals, they enclose only very few of 

 olivine or of magnetite. These minerals, together with the unindivid- 

 ualized substance, are crowded into the spaces between the pyroxenes. 

 In this intermediate space, which surrounds the pyroxene individuals 

 with a continuous network, we find, also, a few small pyroxenes, just 

 as isolated grains of olivine occur in the pyroxene areas. 



A careful examination of this occurrence will, I think, convince the 

 observer that, at the time the pyroxene crystallized, both the olivine 

 and the feldspar crystals, and apparently the magnetite, were already 



