THE CHEETY LIMESTONE. 133 



at times closely intermingled witli the dolomite, so also in the case of this 

 chert rock the silica presents itself in completely crystalline, half crystal- 

 line, and amorphous conditions, these several phases occurring at times in- 

 termingled with one another. In some cases, however, as for instance in 

 the rock at Penokee gap, all of the silica is completely individualized, and 

 the individuals furnished more or less thoroughly with crystal outlines. 

 As seen microscopically, this peculiar quartz rock is perfectly white, stud- 

 ded with minute crystal facets, and of a saccharoidal texture, being often 

 so crumbly as to be readil)^ mistaken for a fine grained sandstone. This 

 impression is confirmed by the crystal facets, which one takes at once to be 

 in the nature of the enlargements of quartz fragments commonly met with 

 in sandstone, but the examination of the thin section fails to substantiate 

 the impression, since the outlines of the original grains are not perceptible. 

 On the whole, then, on account of the gradation varieties between this 

 peculiar rock and those jjhases in which there is more or less chalcedonic 

 and amorjjhous silica, we conclude that here alst) the whole of the silica 

 has separated out in situ. 



The accessory constituents in this chert are few and unimportant in 

 quantity. They include sericite, brown iron oxide, and occasionalh' mao-- 

 netite and dolomite. The usual snow white color of the rock is due to the 

 general absence or sparseness of the iron oxide ingredient. Onh' rarely is 

 the iron oxide present in sufficient quantity thoroughly to i-edden the rock. 

 In these rare cases, however, the chert resembles a jasper. 



In PI. XVI, Fig. 3, we have represented one of tlie finer grained phases 

 of chert as seen in polarized light. The concretionary and semichalcedonic 

 arrangement of the silica is plain, the finer grained portions tending to form 



» 



the centers of ai'eas the outer portions of which and tlie interspaces Ijetween 

 which are composed of the coarser grained quartz. PI. xvi, Fig. 4, shows 

 a section of the chert in which a band of the coai-ser grained quartz is seen 

 adjacent to one of finer, but still completely crystalline, material. Our photo- 

 graphs of these sections of the chert and limestone containing the finest 

 grained and amorphous silica have not proved sufficiently successful for re- 

 production, largely on account of the confused appearance of the objects 

 themselves. 



