OF ARTS AND SaENCES. 259 



much affected that the tests for fusibility, and by etching with acid, are 

 useless. But the smallest unaltered portions of a crystal answer readily 

 to the optical tests, especially if the section is cut thin. By using a cir- 

 cular polarizing quartz plate 3.75 millimetres thick, as recommended 

 by Klein and Rosenbusch, inserted just above the objective, and by 

 turning the analyzer till a violet tint is obtained, the readings can be 

 made with much greater precision than by observing the maximum of 

 darkness. In the presence of the quartz plate, all the double-refracting 

 portions of the section appear in different colors, excepting where a 

 principal section coincides with a uicol plane ; crystals so placed will 

 be violet, and a very slight disturbance will change the color to blue 

 or red. 



The chief defect in this method is in the fact that, used alone, it 

 detects only the feldspar that gives the highest angle, while there 

 may be two kinds present. 



The coarse-grained varieties differ very markedly in external appear- 

 ance. There are several varieties, some of which are common to the 

 most distant portions of the copper district ; they are marked bj' dif- 

 ference in grain, texture, and color, and in the nature of their secondary 

 constituents. This last difference is apparently largely due to variation 

 in the feldspars. The coarser fbrms have more or less orthoclase, and 

 in some this may predominate. The triclinic feldsi^ar varies in char- 

 acter from oligoclase to anorthite. 



The pyroxene is to all appearances the same augitic variety in all, 

 and in all instances it crystallized after the feldspars and occupies the 

 interstices between these. Only in rare instances — as in bed No. 

 95, Eagle River Section — a well-defined cleavage seems to mark the 

 pyroxene as diallage. 



The specific gravity varies from 2.89 to 3.03 ; the colors from dark- 

 brownish black, with resinous lustre, as in bed No. 95, to a very light 

 gray, as in bed No. 107. Between these extremes, there ai-e varieties 

 of many shades of green and brown, or red. Indeed, specimens of 

 the eight or ten varieties differ so widely in external appearance, that 

 it is difficult to believe that they are closely allied to each other, and 

 to the finer-textured rocks, until we see the similarity of constitution 

 and texture as regards primary constituents, and also their similar 

 mode of occurrence, and the fact that they are interbedded with the 

 finer-grained diabase. Such of these coarser forms as exhibit interestr 

 ing phases of alteration will be described. I will remark here that 

 Mr. Marvine, in his otherwise very excellent description of the beds 

 of the Eagle River Section (Geol. of Mich., vol. i. part 2, pp. 119- 



