60 GBEEN MOUNTAINS IN MASSACHUSETTS. 



of white gneisses and those of the eastern core, and again at about the cen- 

 ter of the tunnel, under the central shaft, they come in east of the con- 

 glomerate and fill the eastern half of the tunnel to about 6,000 feet from 

 the east portal, where they are succeeded by the silvery-green schists (Rowe 

 schists) to the east portal. 



Among the perfectly fresh material found at the tunnel dumps a shiny 

 black glistening rock is typical, containing parallel layers of white quartz 

 which thin out and disappear in the rock. These flat lenses are sometimes 

 very irregular and crumpled by large folds or small puckerings. It is found 

 that they correspond to the plane of stratification of the rock wherever the 

 schist is seen in contact with other rocks. The black, shiny part of the 

 rock is filled with sparkling glassy crystals of feldspar, either in imperfect 

 rounded crystals or in simple twins, which contain inclusions of mica, gar- 

 net, etc. The basal cleavage planes are sometimes hounded l»y the brachv- 

 pinacoid (M), the prisms T and 1, and the macrodome, etc., but the crystals 

 are in general rounded or even angular. 



The feldspar twins are according to the albite law, and the crystal is 

 divided into two symmetrical halves, or else the composition-plane is irreg- 

 ular, one half taking up most of the crystal, leaving a small strip to the 

 other. The rock was powdered and the feldspar, separated by the Thoulet 

 solution, analyzed by Mr. R. B. Riggs in the laboratory of the U. S. Geolog- 

 ical Survey at Washington with the following result : 



SiO.2 69-69 



A1,0 3 18-60 



CaO trace 



MgO 0--20 



Na ; 10-28 



K.O 0-40 



Ignition 0-42 



99-59 

 Co, (Combustion), 0-77 0-44C. 



Basal cleavage pieces with the simple twin give an extinction 4° 

 oblique to the twinning-plane and second cleavage (M). Twins measured 

 in the goniometer give angles of 172° 46' to 172° 50' between the basal 

 cleavages of the two twins. The chemical and physical properties are 

 therefore those of albite. These albite crystals vary from large to small; 



