THE QUARTZ SLATE MEMBER. 153 



However tliis mux he, that all of these rucks are nf a fragrmental 

 orioin is rendered sutHcieiith' evident V)v the gTadatinn of those kinds in 

 whieh the fragnieutal texture is lost into those ])hases in which it still 

 remains distinct. It should he said that these g-radations occur constantly 

 within sliort distances, and that those portions in whicli no distinct trace of 

 fragmental origin is now perceptible are all relatively of \('r\' small extent 

 and rare occurrence. It is a noticeable coincidence that the most thor- 

 oughl\' chans'ed varieties met with in this slate ])elt occur in its more 

 western portions, this being at the same time true of the uppermost member 

 of the series. A reason tor this coincidence is later suggested. 



Mieroscop'icul rhdnictiT oj' thr ritrcoiis quartzite. — (Phase 4, PI. xx, Fig's. 

 1, 2, 3, 4.) The vitreous (piartzites' prove, as one would expect from their 

 macroscopic appearance, to lie composed almost entirely of relatively' larg-e 

 sized quartz frag-ments, each one of which has received an enlarg-ement, 

 the several secondar\- enlargenu»nts interlocking with one another in a 

 more or less intricate maimer. Feldspathic quartzites are interstratitied 

 in thin seams at various horizons in the quartz-slate formations, particu- 

 larlv so in the more eastern portion of the belt; but the only occurrence 

 of pure quartzite in this formation, as already noted, is that persistent 

 band which foi-ms throughout the entire extent of the formation its 

 uppermost horizon. The specimens brought from this particular layer 

 furnish some of the handsomest illustrations whicli we have ever met 

 with of a transformation of a sandstone to a vitreous quartzite by the 

 enlargement process.'- The outlines of the original fragments of these 

 rocks are in many cases emphasized by a brown iron oxide, in whicli 

 case the rock has usually a more or less distinctly brownish tinge; but in 

 I'nanv cases the emphasizing mineral is chlorite in minute Hakes. In the 

 latter case the rock is either of a light gTa^- color, or, if the chlorite is 



'As used in this memoir, the word "quartzite" is restricted to rocks which have been derived I'roiii 

 fraguient.ils. Tlie fundamental ditl'erourc which exists hctwcen rocks of this kind and those in 

 wliidi tlie ([uartz is an original crystallization, we lielievc, is in 1 his volume for the first time fully rec.ijf- 

 nized. However c|uartzite-like the uonfragraental (piartzose rocks of the Iroii-hcariuf; and C'lierty 

 limestone members are, they are .ihvays designated by some other name than iiuartzite. 



-For a more complete exi)lanation of this enlargement jirocess, with illustrations drawn from 

 very many localities ami from dilft rent geological horizons, see Hull. 1'. S. tleol. Survey No. 8, by 

 R. D. Irviu" and C K. Van Hise. 



