358 GEOLOGY OF TUE EUREKA DISTRICT. 



crystuUizatiou appears luore. reaililj^ altered than the luacvo.scopic primary crystals, 

 and when discolored by u'oii oxide forms dark red, narrow borders around the still 

 fresh larger augites and black bordered hornblendes, suggesting the characteristic 

 black border of the latter mineral, from which, however, it is easily distinguished. 

 Aggregations of augite crystals around a foreign nucleus are occasioually met with. 



The hornblende of this rock is quite abundant in crystals, which are not very 

 well developed, except in the prism zone, where, besides the ordinary faces, ooP and 

 ooP do, there is occasionally the orthopinacoid, ooP do. The terminal faces are not 

 recognizable iu the thin sections studied, but, judging from the macroscopic crystals 

 in the hand S]iecimens, they appear to be those usually developed. The majority of 

 individuals seen under the microscope are irregularly outlined. The largest reach 

 4 to 5™"" iu length, but the greater number average less than 1™°'. They do not take 

 part in the composition of the grouudmass. The cleavage is very perfect, parallel to 

 the prism, forming a very sharp network of parallel lines, thus differing from the 

 pyroxene, in which the less perfect cleavage is combined with irregular cracks. There 

 is in some instances a second cleavage, parallel to the clinopinacoid, never well devel- 

 oped. Some of the individuals are t^vinned in the usual manner parallel to the 

 orthopiuiicoid. The hornblende is dark reddish brown in color, with a strong absoriD- 

 tion. In the dark and more resinous varieties of the audesite (Nos. 77, 78, 79) the 

 color is reddish brown, being dark brown parallel to the axis of least elasticity (c), 

 nearly the same shade of brown parallel to the axis of mean elasticity (b), and light 

 yellowish brown parallel to the axis of greatest elasticity (a); that is, c = dark brown, 

 b = dark brown, a = yellowish brown, and c = b > a, possibly c > b > a. In the lighter 

 colored, purple and fissile varieties of the andesite (Nos. 85, 86) the pleochroism is 

 greater, but the absorption less, the brown color having a greenish tinge and the 

 pleochroism being asfoUows: Parallel to c browish green, parallel to b reddish brown, 

 l)arallel to n yellow, and c = b>a. In the specimen from Trail Hill (No. 90) the color 

 parallel to c is brownish red, parallel to b brown, parallel to a light brown, c > b > a. 



The hornblende individuals are surrounded by an opaque black border that 

 bounds the whole outline of each section, the fractured or eroded portions in the same 

 manner as the crystal faces; its width varies somewhat, and is not constant for any 

 cue individual. It is quite sharply defined, both on the outside and inside, though 

 occasionally it is seen shading into the hornblende substance as minute opaque dust. 

 It appears to be magnetite, having the same luster iu incident light and the same 

 products of decomj)osition, hydrous oxide of iron. Spots of similar magnetite dust 

 occur inclosed in the hornblende, besides the inclusions of coarser grains and crystals 

 of magnetite, sometimes arranged in lines parallel to the clinopinacoidal cleavage. 

 The fact that the black border does not occur between the hornblende and feldspar 

 or augite when they are in contact, but always between hornblende and the ground- 

 mass, together with the fact that it surrounds the fractured portions and lines the 



