216 THE MAKQUETTE IKON BEAKIXG DISTIiK.'T. 



masses of bmwu rutile, and a few rlioinboliedra of some almost colorless 

 carljoiiate. The cliloritoid is the most interesting component. It is in 

 large tabular plates with a cellular structure, and is filled with inclusions of 

 quartz, rutile, and portions of t!ie rock's groundmass. As usually seen, the 

 plates appear as prisms with a distinct cleavage parallel to their long direc- 

 tions, and sometimes a parting perpendicular thereto. In the direction of 

 the cleavage their color is a deep bluish-green, and perpendicular to it a 

 pale yellowish-green. Between crossed nicols the prisms are all striated 

 with longitudinal twinning lamella?, whose extinctions, measured against 

 the cleavage lines, vary between 1° and 21°. The prisms, of course, ai'e 

 vertical sections of the plates, whose cleavage is parallel to the base. 



Evidently tlie chloritoid is the youngest mineral in these rocks. Not 

 only does its contact structure indicate this fact, and the position of its 

 plates with respect to the foliation, but the same mineral in well-developed 

 plates of the same hixhh is foimd not only in the bowlders of the gneisses 

 in the conglomerates near the Piatt mine, but as well in the matrix of these 

 rocks. 



Other specimens of the gneisses differ from this one mainly in the size 

 of the chloritoid plates. In some the plates are very large, and in others 

 they measure only a few tenths of a millimeter in their longer directions. 

 In one or two cases the chlorite appears to be in bands in the schists, other 

 portions of the rocks Ijeing without them. Usually its plates are dissemi- 

 nated irregularly. 



In two or three sections there were also noticed a few small, ill-defined 

 prisms of dark greenish-blue tourmaline, a mineral whose presence in rocks 

 is usually ascribed to contact or fumarole action. In the present instance 

 there is no evidence of any kind to indicate that the mineral is of contact 

 origin. Its grains are distributed irregularly through the gneisses, without 

 any reference to their foliation, and the mineral is consequently subsequent 

 in its origin to the production of the gneissic structure. 



COMPOSITION AND ORIGIN. 



The similarity of the matrix of tlie Palmer gneisses to the altered 

 mosaic of the crushed granites and to the altered feldspars of the more 

 massive phases of these rocks, and the discovery of indefinitely outlined 



