LITIIOLOGY. 151 



light their color is brownish red. The feldspar shows its characteristic 

 bandings in polarized light, but is usually troubled and clouded by de- 

 composition. The bandings are sometimes almost obliterated on account 

 of alteration, and then the crystals do not become dark in any position 

 between crossed Nicols, but show only the effects of an aggregate of 

 secondary products. Crystals or grains resembling magnetite are found 

 intact ; but often a black rounded kernel or skeleton is found in a gray- 

 ish mass, which is recognized as characteristic of the decomposition of 

 titanic iron. Very minute needles of apatite, which pierce through the 

 other minerals, and which are often grouped together in large numbers, 

 are invariably present. Other products have also resulted from decom- 

 position, a characteristic one of which is epidote. This is often macro- 

 scopically seen filling amygdaloidal cavities, but is more often seen with 

 the microscope, especially in sections that show a much altered feldspar. 

 It appears as a very light yellow, slightly dichroic product, often in very 

 minute particles which in polarized light assume the most brilliant colors. 

 Calcite is rarely absent. It fills the cavities and pores, and, when not 

 visible with the microscope, a specimen of the rock when moistened with 

 acid will effervesce, and indicate its pressure. Biotite is not rare, but 

 whether as an original or a secondary product is not certain. Quartz is 

 not uncommon, but it is plainly a secondary product. These are the 

 most general characteristics of the first well marked variety, which may 

 be called the chloritic type. 



There are some large dykes of this rock on the road from the Glen 

 house to the summit of Mt. Washington. In sections of specimens the 

 double system of twinning, which so often characterizes labradorite, is 

 conspicuous. A section of a specimen from Stark indicates that the augite 

 is almost entirely gone, and chlorite takes its place, fills cavities in the 

 rock, and forms little concretions in spots. A rock near the Sagamore 

 house resembles the last, but in it much more epidote has been formed 

 and less chlorite, and, as is usual in such cases, the feldspar has suffered 

 more and the augite less. This rock also contains pyrites. A rock 

 that forms a dyke which cuts the gabbro on Mt. Washington river is 

 remarkable for its large content of pyrite and magnetite. The augite is 

 in small grains, and has been altered in part into chlorite, and in part 

 into hornblende. The decomposition of a light-colored diabase has 



