ANDESITE. 353 



Microscopical Ortboclase (sauidine) and plagioclase seem nearly equal in impor- 

 tance. Both are very fresh and in most cases contain few interpositions, although a 

 few crystals carry a very large number of devitrifled inclusions. Hornblende in yel- 

 lowish-green individuals is quite plenty beside the biotite and both minerals are 

 fresh. The amount of quartz seems limited to the macroscopically visible, rounded 

 grains, and these, by their freedom from all inclusions and worn appearance, seem like 

 accidental rather than normal constituents of the rock. Their number is small, and 

 even if original it seems more proper to consider them as accessory. A silica deter- 

 mination of an average specimen yielded but 61.22 per cent., so that it is evidently not 

 to be classed with the acidic group. Magnetite is abundant, as well as pale mineral in 

 irregular oblong grains, which may be titanite. Apatite is inclosed in all the larger 

 elements excepting the quartz. 



The grouudmass is microfelsitic in large degree and contains few crystalline par- 

 ticles. It shows a distinct fluidal structure, made plain by the contrast between the 

 portions carrying indistinct brown needles and colorless portions. The needles are 

 sometimes grouped in an imperfectly radiate manner about some small crystal, in a 

 manner similar to that in felso-spherulites. These act feebly on polarized light, giving 

 a faint black cross when seen between crossed nicols. Some colorless isotropic spots 

 seem to be glass. The movements which produced the fluidal structure are also indi- 

 cated by the crumpled biotite leaves and broken hornblende prisms. 



ANDESITE. 



Andesitic rocks have not been found within the limits of the Mosquito Eange 

 map, but at the Buffalo Peaks, a few miles south, a large variety occurs. In the course 

 of a hurried trip a number of specimens were collected from this locality, represent- 

 ing several types; of these, two are sufficiently marked in character and occurrence 

 to merit particular notice. 



PYROXENE-BEARING HORNBLENDE- A.NDESITE. 



Macroscopical The rock which in the form of a sheet caps the mountain is a pro- 

 nounced hornblende-audesite [143]. Macroscopically it is dark brown in color and 

 contains feldspar in clear or ashen-gray crystals and dark, glistening hornblende 

 prisms as most prominent constituents. With the aid of the lens green prisms of 

 pyroxene and ore particles are quite abundantly visible. The brownish grouudmass, 

 which gives tone to the whole rock, is rather more abundant than all the crystals 

 together. 



Microscopical Under the microscope the hornblende is found to possess the usual 

 characteristics of that mineral in such rocks. It has a dark, granular border, or is 

 occasionally entirely replaced by a mass containing opaque ore grains, augite prisms, 

 and some calcite as secondary elements. Besides the hornblende appear both hypers- 

 thene and augite, in smaller crystals, but more numerous. The former of these 

 minerals possesses the same characteristics as in the accompanying hypersthene- 

 audesite. Most of the feldspars are distinctly plagioclase and some of them contain 

 irregular glass-inclusions in great number, many of which are now much devitrified. 

 Magnetite and large dusty apatite prisms are sparingly present among the porphy- 

 ritfcally imbedded crystals. 



MON XII 23 



