OP ARTS AND SCIENCES. 279 



zontally, and so closely as to obscure the brush-structure. I can think 

 of no better illustration. In places, this structure exists without any 

 visible black rods. 



Where the augite is clouded by a beginning change in the interior, 

 we find these planes of bluish particles and the brush-fence structures 

 to be starting-points, having, in those representing the first stages, only 

 slightly ditfuj^ed yellowish-green substance ; in others, the primary inter- 

 positions become more and more obscured, and the planes and arrange- 

 ment of bottle-green substance are wholly lost in the green mass which 

 penetrates with a serrated front into the fresh augite. This green sub- 

 stance also attacks the crystal from the outside, and along the cracks. 

 Under a high objective the contact resolves itself into a jagged line. 



A considerably altered augite crystal is surrounded by a green mass, 

 and more or less broken up by cross-bars of the same substance, while 

 the interior of the remaining augite fragments is often occupied by 

 masses of the green substance which had the interpositions for starting- 

 points. 



This alteration product is yellowish-green where very tliin, and trans- 

 lucent dirty-green or grayish-green where thicker. It is composed of 

 countless minute stars, indicating radiating structure, and is double- 

 refracting, but shows no absorption for color, and little or none for 

 intensity. The same substance occurs, to a slight extent, in the ruined 

 feldspar with the chlorite, which it sometimes borders. 



The green product is associated with magnetite in large and small 

 panicles, and, where the change progressed along a crack, the green 

 band has a median line of more or less closely contiguous films and 

 grains of magnetite, which corresponds with the structure, as observed 

 in reflected light on the uncovered sections. 



In the rock from the Eagle River, district 94, the augite contains 

 only isolated particles of red ferrite, and then, generall}'^, only in the 

 planes of bluish interpositions, and where these are near the altered 

 portions. But in an almost identical rock, coming from the same zone, 

 north of the Delaware mine, and in which the augite seems to have 

 been wholly changed, an examination in reflected light shows the mag- 

 netite surrounded by an opaque mass of red oxide, which, further from 

 the magnetite, resolves itself into a cloud of minute particles of trans- 

 parent brown, or red ferrite suspended in the green alteration product. 

 Optical measurements on sections of feldsjjar in the zone 0: li in the 

 preparations of this rock gave the same result as in bed 94. 



The abundant apatite needles are characterized by their hexagonal 

 sections, or, where cut longitudinally, by their very feeble double-refi-ac- 



