276 PROCEEDINGS OF THE AMERICAN ACADEMY 



lur;ier crystals are almost wholly free from striation. They are flesh- 

 red from having begun to change. The spaces between these crystals 

 have dark-green and black colors, and, under the microscope with a low 

 power, are found to consist, in part, of a grayish-green, compact min- 

 eral, with semi-conchoidal fracture, very soft under the needle-point, 

 and containing veins and fragments of a brilliant black mineral, which, 

 from its streak, and action under the magnet, appears to be magnetite. 

 Other spaces consist of a lighter green mineral, with radiating fibrous 

 structure in segments of spheres, and very soft under the needle. With 

 this light-green mineral are associated aggregated crystalline grains of 

 quartz. 



Examining different parts of the specimen, all the constituents are 

 found to envelop long, slender, hexagonal prisms of limpid white 

 apatite. 



In uncovered thin sections, under the microscope, we find that the 

 red color of the feldspar is due to a staining on the sides and along 

 the planes of twinning and in fissures. Where the crystals have not 

 become sufTu-iently altered to be unrecognizable as such, they are not 

 scratched by the needle. 



Within the spaces between the feldspar crystals are isolated rem- 

 nants of augite, in places still transparent, fresh and smoky-brown, but 

 generally surrounded and veined with a slightly translucent grayish- 

 green substance, which is very soft under the needle, and contains veins 

 and grains of magnetite : it is the compact, soft, green mineral men- 

 tioned above. The magnetite is here porous in structure, and, when 

 scratched with a magnetized needle, gives a shining black streak, and 

 leaves particles adhering to the needle : facts which indicate that it 

 forms a network of filmy veins intersecting the soft, green substance 

 in all directions, like the cement of a breccia. 



The light-green mineral with radiating structure is here seen to be 

 entirely distinct from the grayish-green substance ; it is very soft under 

 the needle, and contains no magnetite, but is associated with quartz. 



In polarized light some of the feldspar crystals are found to be 

 beautifully marked throughout with thin twin striae ; but these are 

 generally either absent, or ordy two or three narrow ones are associated 

 with very bi'oad unstriated bands. 



Two systems of twinning occur among the feldspar crystals, cutting 

 each other nearly at a right angle. In the greater number of instances, 

 the composition plane is the plane of symmetry ; in these double twins, 

 a long, sharply defined crystal will have broad areas free from longi- 

 tudinal striie, and apparently of orthoclase, with a kw longitudinal 



