DIORITE. 343 



grows more gueissoid from contact influence, but this is not marked. The 

 rock is normally dark-gray or nearly black, with a shade of brown, and 

 seems at first sight to be fine-grained ; but when held to the light it is seen 

 to be made up of squarish surfaces, from a half to three-fourths of an inch 

 across, of jet-black to dark-green hornblende, very beautifully luster- 

 mottled by fresh, white, striate, broad lath-shaped 2)lagioclases, and show- 

 ing rarely a grain of quartz, garnet, or a black ore. 



It is in places bedded, and on the west, in the hill above Cooleyville, 

 one traces the amphibolite into immediate proximity to the diorite, where 

 it is thickened unusually, is massive, and gi'eatly resembles the diorite. 

 It may be a compacted and altered ash bed, associated with the eruptive 

 rock. In the southwest portion of the mass, near A. Pierce's, in Prescott 

 Hollow, the diorite is a coarse, white, feldspathic, slightly saussuritic rock, 

 with only small, distant patches of a dark silicate, now changed to a mixture 

 of actinolite and biotite. 



The freshest material for microscopical study was obtained from a 

 great bowlder ou the north side of the road west from Prescott Center, near 

 the last house in the village. (See PI. Ill, fig. 3.) It presents a very 

 attractive appearance under the microscope. A portion of a single horn- 

 blende crystal occupies the whole field, notched by the regular crystals of 

 feldspar, which run in every direction. It shows a maximum extinction 

 angle of 22", and is therefore near labradorite. It is quite fresh, and full 

 of acicular needles. 



The hornblende is deep-green, extinction 20°, with slight pleochroism, 

 c=b <a; c = blue, ij — olive-green, a = yellow. 



It is dusted full of a very fine black powder, in bands more or less 

 dense, parallel to the cleavage, rendering wholly opaque the central parts 

 of the lobes, into which the crystal is divided by the feldspars, while it 

 shades off toward the border, where it is still more densely accumulated 

 in a broad band bordering the feldspars. In places the central portions 

 of the lobes are crowded by red-brown scales, placed largely at right 

 angles to the bedding and resembling those found in bronzite. Cleavage 

 pieces of the hornblende measured with the reflecting goniometer gave 

 124° 30'. Rarely a large pale-green pyroxene appears, with a border 

 of hornblende ; and meuaccanite and red-brown rutile, with fine leucoxene 

 borders, are present. 



