THE LEYDEN AEGILLITE. 209 



Besides these exomorphic effects of the contact, the tonaHte shows 

 distinctly an endomorphic influence of the schist upon itself It is finer 

 grained than usual, though it is granular to the eye, and the deep flesh- 

 red feldspar stands out on a background green from the abundance of 

 chlorite. It is rudely foliated, and the foliation surfaces are dull-green, 

 like the schist itself, and in transverse sections the microscope reveals thin, 

 wavy layers, winding in between thick layers of the feldspathic material, 

 which seem to be made up of the sericitic matter from the scliist crowded 

 into the fissures. 



The main mass of the argillite followed north from the contact retains 

 all the complex contortions common in the rock, but it is soaked full of 

 quartz, or quartz and feldspar, the parallel bands being in some cases sepa- 

 rated as much as 30°"° by the intrusion of these new constituents. There 

 is also much coarse muscovite, and the rock is in places greatly brecciated. 

 When it is followed farther north small staurolites appear and the next 

 band is reached. 



Going a short distance west along the road to the bridge over West 

 Brook, and then south 165 feet along the brook, one finds a fine contact of 

 the argillite and the tonalite exposed. The rock is here more arenaceous, 

 and is indurated to a homfels. 



THE CHIASTOLITE-SCHIST. 



This rock (see fig. 1, PI. III.) may be studied most conveniently on the 

 southeast slope of Belmont — a great symmetrical drumlin, bare of trees, in 

 the northwest corner of Hatfield. The original bedding of the rock is here 

 clearly marked by bands of sandstone about an inch wide, separated by 

 argiUite layers of twice this thickness. The whole is extremely contorted, 

 and the well-marked cleavage oyersprings the sandy layers in almost every 

 case. The original clay layers are now a fine, dark-gray mica-schist, to 

 which one would hardly still apply the name argillite, and in some places 

 it is coarsely muscovitic. The schist is full of chiastolite crystals, square 

 prisms about 4™™ wide and 40°'"' long, enfolded ui the layers of the schist, 

 as is Usual with this mineral. These are now uniformly changed into a 

 shining-white muscovite in matted scales (with traces of the black cross 

 everywhere remaining), in which small andalusite crystals occur so abun- 

 dantly as often to occupy half the space. They are in stout prisms nearly 



MON XXIX 14 



