THE MONA SCHISTS. 155 



grain. Occasionally their texture is so fine that hand specimens resemble 

 greenish cherts in appearance, or massive graywacke-like sediments. In 

 the ledo-e the rocks present a inidely schistose structure, which is lost in the 

 specimens. In some exposures, as in the knob in the NE. \ NW. ] sec. 

 28, T. 48 N., R. 26 W. (Atlas Sheet XXX), the rock is divided into oval 

 or lenticular masses, separated from one another by schistose material of 

 the same nature as that composing the oval masses, but of nuich finer grain. 

 This structure, as has been pointed out by Williams, is neither concretionary 

 nor ao-o-lomeratic. It is similar to the structure of certain Saxon schists 

 which Rothpletz has shown to be mechanical in origin. 



In thin section the aphanitic schists are found to be nearly as uniform 

 in composition as they are in appearance. They consist of granular 

 epidote, small flakes and needles of cldorite and hornblende, and altered 

 plagioclase, with the addition usually of calcite, leucoxene, a little quartz, 

 and mosaic areas of albite and quartz. The plagioclase may sometimes be 

 detected in small lath-shaped crystals, lying in all azimuths amidst the 

 other components, but more frequently the mineral is so much decomposed 

 that its orio-inal form can no longer be recogiiized. The epidote grains are 

 usually scattered through the slide. Not infrequently, however, they are 

 aggregated into little groups with the cross-sections of feldspars. The 

 plates and needles of chlorite and the needles of hornblende, which are 

 rather abundant in some sections of the rocks, are quite small. They 

 are intermingled with a few sericite flakes, a little calcite, and small areas 

 of the clear mosiac already referred to. Usually these ct)nstituents inclose 

 the leucoxene and the altered plagioclase crystals in the same way as 

 glass incloses the crystal components in a glassy basalt. In other cases the 

 use of crossed nicols brings out an arrangement of the various constituents 

 in such a way as to resemble the structure of fine-grained diabases, and 

 even of gabbros. In still other instances, in the apparently heterogeneous 

 aggregate of components, under crossed nicols a structure resembling that 

 of tuff is discerned. Broken pieces of altered plagioclase are discovered 

 in a fine-grained matrix with no well-defined structure. In composition 

 the greenstones are altered diabases or basalts, and their structure, when 

 discernible, is either that of basic lavas or that of tuft's. 



