192 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



crystals of titanite, wine-yellow and colorless, in considerable number, 

 which sometimes contain grains of black ore, thick prisms of rutile, often 

 with dark border, rust, and a little coaly matter. The centers of the horn- 

 blende blades are often red-brown, the outside green, and they seem some- 

 times to be built up around plates of biotite, but more often the color shades 

 off indefinitely at the edges, and is peculiar to the hornblende. The horn- 

 blende has weak pleoclii-oism jc>lj>a; c = gi'eeu, b = olive, a = yellow. 



The large white porphyritic sjjots, 2-3™" across, are so loaded with 

 opaque white dust, muscovite scales, etc., that it is generally only possible 

 to make out a mosaic of untwinned feldspar and quartz grains, and, in 

 the absence of cleavage and twinning, to make sure that the mineral is in 

 part biaxial. In one large grain, cut parallel to M (010), an optical axis 

 emerged at the lower left-hand border, indicating auorthite, and where 

 twinning occurred the extinction angle was very large, gi^^ng the same 

 indication. 



(d) Rim of a similar ^^ anvil" from Plainfield. (In the collection of 

 Amherst College. See PI. V, fig. 1, p. 302, for section.) In the matted, 

 green, fibrous hornblende, greatly darkened by rust and coal dust, are 

 many scales of a greenish mica, garnets with the same radial inclusions as 

 in the West Chestei-field schist (p. 182), curious long red prisms of rutile, 

 matted fine white needles with longitudinal extinction, apparently zoisite, 

 and a fine plagioclase, extinction 26°, loaded with coal dust, but with clear 

 border. In other cases sections cut at right angles to both cleavages gave 

 an extinction of 38° to 45°, indicating a A^ery basic feldspar 



It is significant, as connecting these beds with the porphyritic amphibo- 

 lites, that rounded clear spots of impure plagioclase appear, from which all 

 the dark constituents are excluded. 



The slides of black hornblende-schist or amphibolite last described, 

 cut from the thin plates of the rock which borders the limestone beds, and 

 which have manifestly been dejived from the limestone, fm-nish abundant 

 proof that some amphibolite beds may originate from limestone. 



The thin beds of amphibolite of exactly similar habit with the above 

 and found in the Conway schists have clearly the same origin, the change 

 having reached the center of the former limestone from each side. These 

 beds have commonly a thickness of 6 inches to 1 foot. 



(e) The amphibolite at the brook crossing in Whately. (See PI. V, fig. 3, 



