HOOSAC MOUNTAIN. 63 



chlorite has the appearance of an alteration product of the biotite. 1 When 

 the chlorite occurs independently in stout plates it has a marked pleochroism 

 varying from green to yellow green, an extinction several degrees oblique 

 to the cleavage and twinning with OP as composition-plane. Tourmaline 

 and apatite occur in imperfect prisms, magnetite in octahedra, and rutile in 

 small crystals, often with the heart-shaped twins. 



In several specimens a little ottrelite has been noticed, and at one local- 

 ity this mineral occurs in such amount that the rock must be called an 

 ottrelite-schist. This is interesting in that it still further proves the litho- 

 logical identity of the Hoosac, Greylock, and Berkshire schists, since this 

 mineral is found in all three of these formations. The hand specimen is a 

 shiny, greenish schist containing crystals of garnet and dotted with little 

 black ottrelite crystals. In the slide the ottrelite occurs in comparatively 

 large crystals with the characteristic indigo-blue, yellow, olive-green ple- 

 ochroism. The extinction is several degrees oblique to the cleavage; it is 

 twinned parallel to the base, and basal sections give a faint bisectrix. It 

 occurs associated witli irregular masses of black ore; a number of small 

 prisms of ottrelite surround a plate of the ore (ilmenite?). Plates of mus- 

 covite and a \\-\v grains of quartz compose the rest of the rock. The 

 ottrelite is rilled with little prisms of rutile with the "knee "-twin. Basal sec- 

 tions show the blue color, with vibrations parallel to h (at right angles to 

 the axial plane), and the yellow green parallel to a; hence it has the ple- 

 ochroism of most ottrelites. 2 



In this schist we recognize no elastic element with certainty and the 

 feldspar, quartz, micas, etc., appear to have formed contemporaneously, for 

 the feldspars contain inclusions of the other elements and in turn are some- 

 times crossed by tongues of mica and quartz. 



While the term "schist" is applied to this rock owing to its frequent 

 coarsely crystalline character, yet its great similarity should lie noted to 

 crystalline rocks described from Germany and elsewhere as albite-phyllites, 

 which contain porphyritic albites with similar inclusions, micas, magnetite, 

 etc. 



'This association of biotite and chlorite is common in the hydroniica. schists of the Ureen moun- 

 tains ami is often suggestive of hydration by weathering. 

 - Cf. Rosenbusili: I'liysiographie, vol. 1, p. 494. 



