THE CONWAY SCHISTS. 193 



p. 302, for section.) The amphibolite at the brook crossing is thi-ust forcibly 

 up tlu'ough the argilHte, togetlier with the black Conway limestone, as 

 described on page 196. It shows in many ways a transition between the 

 narrow bands of hornblendic rock which form selvages to the limestone 

 beds, as described above, and the larger bed which is the subject of the 

 next chapter. 



It is a dull, dark-green, massive rock, which shows with a lens the 

 usual interlacing network of actinolite blades, with rare open white spots 

 composed of a granular feldspar, much changed to mica The feldspar is 

 optically positive and has extinction +6^° on M (010), and so is an oligoclase. 



The hornblende is peculiar in two ways. It has a brown center and 

 grades through green to colorless at the ends of the blades The brown 

 is like the cummingtonite found in the Conway schists farther west. It 

 has low absorption colors. The brown shows c = greenish brown, h = red- 

 brown, a = pale brown; the green, c = blue-green, b = pale green, a = pale 

 yellow: jc>1j = a. The blades are fibrous and often twinned, and give 

 extinction 14° to 17°. 



The second peculiarity of the hornblende blades is that the brown 

 centers often show dark-brown bands situated in the ba.sal parting and 

 sending out long, straight needles in both directions parallel to the vertical 

 axis, which makes them look like combs with teeth directed both ways. 

 These straight needles are also abundant everywhere in the hornblende 

 and in the feldspars and seem to be rutile. A "hof" surrounds the larger 

 comblike accumulations and dims their outline 



Other hornblendes are built up around red biotites filled with coaly 

 matter. A few grains of calcite occur, and the black ore grains show no 

 trace of leucoxene. 



This agrees so nearly with the calcite-derived amphibolite described 

 above that one must assign to it the same origin. Its close association 

 with the limestone strengthens this conclusion. It is, however, not cer- 

 tain that this bed is part of the large bed next described, though highly 

 probable. 



(/) Tlie great Whatehj atnpliibolite. Tliis bed, which extends as a broad 

 band across Whately and Williamsburg, is for the most part a very fine- 

 grained, black, fissile rock, and in sections cut from the north end of the 

 bed the hornblende is present in a network of long blades with strong 



MON XXIX 13 



