PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 205 



Diabase.] 



These results sufficiently show that the mineral is augite, in the rare position 

 of being cut nearly perpendicular to a bisectrix (), thus bringing two cleavages, 

 one of which is rarely seen in augite, at right angles to each other in the same 

 grain. 



One section examined, the same as that examined by Dr. Wadsworth. 

 Remark. Another section having been made of rock No. 133, the dubious char- 

 acters do not appear, but the characteristic cleavages of augite are evident, as below, 

 in which, along with the prismatic faces 110(M), may be seen also the pinacoidal, 

 OK) and 100. Those parallel with the prism faces make an angle of nearly 90, and 



those parallel with the piuacoids also form a right angle, the 

 section being cut about perpendicular to the prism. These 

 augites are of older generation than the feldspar, or cotem- 

 porary with it, and they tend toward diallage more strongly 

 than those of later origin at least only the prismatic cleav- 

 age appears in the ophitic augites, usually, but in the idio- 

 morphic ones the pinacoidal cleavages are often prominent. 

 FIG. u. PRISMATIC SECTION Sometimes they are completely surrounded by a portion of 



OF AUGITE IN NO. 133. 



the non-differentiated magma, there being no appearance of 



decay in the augite, but the line of separation between it and the magma being per- 

 fectly distinct and sudden. The same is true when this augite comes into contact 

 with the feldspar. 



The altered residuum of the nuii/inu is green, embraces all the other minerals, and 

 is massive, giving a dense or " felted " polarization which renders it nearly opaque 

 between crossed nicols. No fibrous structure is visible. It is probably some form 

 of chlorite. 



There is also in this sectios a mineral which is brown, and, according to Prof. 

 ' Lacroix, resembles boidinyite. (Mineralogie de France, part I, page 442. Compare, 

 also, Mineral ogical Magazine, vol. i, page 154.) In the original description of bow- 

 lingite, by Hannay, it is said to be green "of a fine, deep green color by transmitted 

 light." Lacroix connects this mineral with the mineral iddingsite described by 

 Lawson (Bulletin of Mining, California, i, page 31, 1893) which is usually a dark 

 chestnut brown, but occasionally green. 



The DKKjiK'tite in this rock may be primary, as there is no decay (or but very 

 slight) visible in any of the elements, yet its form is very irregular. It embraces 

 idiomorphic feldspars (compare figure 4, page 99) and small quantities of the devit- 

 rified glassy magma. Some stout apatites are cut perpendicular to the vertical axis, 

 and others are included in the larger feldspars. 



Aye. Cabotian; Beaver Bay diabase. N. H. w. 



