PLATE V. 



SECTIONS OP AMPHIBOLITES DERIVED FROM LIMESTONES. 



Silurian and Devonian age. Drawn with lower nicols to show the pleochroism of the hornblende. 

 The upper nicol is used to bring out the outlines of the colorless mosaic. The plane of polari- 

 zation is parallel to the long side of the plate. The hornblende crystals are generally full of 

 colorless grains like those of the ground. All show remnants of calcite. X 20. 



Fig. 1. ^Calcareous garnet-amphibolite. Couway schist, Plainfield. From the base of the large 

 "anvil." figured in PI. XXXIII and described on page 191, and formed by the metamorphism of 

 the impure limestone which still makes the shaft of the anvil. The rock has the aspect of a 

 coarse hornblende-schist, but effervesces with acid. The coarser-grained portion of the color- 

 less mosaic is clastic, identical with that of the adjacent mica-schist, and the garnets have the 

 same symmetrical arrangement of the coaly impurities. The finer part of the colorless ground 

 is plagioclase. One large grain is marked by triclinic striation and most of it is crushed. The 

 biaxial character of many grains could be determined. Extinction 37°, indicating anorthite. 

 The ragged hornblende plates inclose many colorless grains, generally quartz or calcite, but 

 sometimes small colorless zircons with deep halos of darker color. The dull portions, heavily 

 dusted ^vith carbon grains, are remnants of corroded calcite. Swarms of leucoxene grains 

 surround ilmenite. (See Analysis I, p. 303.) 



Fig. 2. — Amphibolite. Whateiy. From bridge west of the Whately Hotel. A black, massive 

 amphibolite; forming portion of limestone bed in Conway schist and folded into argillite. 

 The long hornblende blades are often centrally brown, with colorless ends, and loaded with 

 transverse black bands which send out comb-like teeth parallel to the vertical axis. There is 

 some biotite. Red rutile surrounds black ore. and small colorless zircons (?) appear, surrounded 

 by dark halos. There is a mosaic of feldspar without twinning and generally without cleavage. 

 (See pp. 192, 196.) 



Fig. 3. — Amphibolite. Bemardston, near R. Park's. From the Devonian Bernardston series. A 

 black, massive rock, made of broad, stout, interlaced blades of hornblende. These blades 

 grade into radiate tufts of horn))lende needles. The hornblende crystals inclose many color- 

 less grains, often of branching and irregular forms, resembling the grains of titanite which 

 surround some of the black ore masses in the slide. Some of the grains of the colorless mosaic 

 are twinned and have generallj' the small extinction angle of albite. Others show secondary 

 growths around rounded centers. One fine fibrous and punctate fragment seems to be organic, 

 resembling a brachiopod shell. It is too small to show in the drawing. (See Analysis VII, 

 p. 303, and comparative discussion, p. 27.5.) 



Fig. 4. — Garnet-graphite-amphibolite. Bowlder from Leverett, but coming with great probability 

 from one of the coarser Bemardston beds or from the great Guilford bed in the Conway 

 schist, figured on PI. VI (fig. 2). A complete block of massive amphibolite. The stout 

 interlacing blades of hornblende contain in their meshes little granular mosaic, which 

 decomposes readily and gives a beautiful surface. The fresh surface effervesces. A few gar- 

 nets appear. The striking peculiarity, indicating the derivation of the rock from a graphitic 

 limestone, is that the surface of very many of the hornblende crystals show shining scales of 

 graphite, which only rarely appear in the photograph, x +. 

 302 



