284 PROCEEDINGS OF THE AMERICAN ACADEMY 



more or less associated with red and black iron stains, which are pos- 

 sibly the source of some of the magnetite. 



The plagioclase retains in many places its twin-striation in polarized 

 light, but, even where freshest, it contains many tufts of radiating fibres 

 or laminas of chlorite, and the sections show that a large part of the 

 feldspar has been changed to pseudomorphs and pseudo-amygdules of 

 chlorite. Optical measurements on random sections in the zone 0: 

 ii give only low angles that may belong to either all)ite or oligoclase, 

 here undoubtedly the latter. The specific gravity of the rock is 2.73, 

 wliich also points to oligoclase. 



The external appearance of the middle zone differs from that of the 

 lower in having a rather coarse grain, and in that the feldsjjar crystals 

 are pink and of all sizes, from ^ inch down, while the very irregularly 

 shaped masses of dark green chlorite often reach J inch in diameter. 



In thin sections from the middle zone, I found the pyroxene wholly 

 represented by its characteristic pseudomorphs with iron-stained 

 cracks ; the plagioclase is also much more altered. 



The formation of the chlorite pseudo-amygdules after the manner 

 already described is beautifully illustrated in this rock. 



This chloritic substance appears, under a glass, both as a compact 

 and as a very finely scaley, dark-green mineral. The hardness of the 

 compact jjortions is 2.0. It fuses at 3-3.5 to a dull black magnetic 

 globule. It dissolves in muriatic acid, leaving pulverulent silica, and 

 the solution contains alumina, protoxide of iron, and magnesia ; an ap- 

 preciable amount of both potash and soda was found, both after boiling 

 in water and in the acid solution. (See complete analysis of this 

 mineral.) 



In the thin sections, the radiating laminai are decidedly dichroitic, 

 being straw-yellow when the longer direction coincides with the longer 

 diagonal of the nicol, and bluish green when parallel to the shorter 

 diagonal. Between crossed nicols it seems to be uniaxial, for the 

 crystals become dark when the longer direction coincides with a nicol 

 plane, and portions were found which revolved dark. Its appearance 

 in polarized light corresponds very closely with that of a section of 

 diabantite* which Mr. Hawes kindly sent me; but it differs in its 

 hardness, diabantite being only 1. 



The following analyses (made for me by Mr. Woodward, in the 

 Laboratory of the SheflSeld Scientific School) are of specimens from 



* On Diabantite. Geo. W. Hawes, Amer. Jour. Sci., June, 1875, p. 454. 



