MICACEOUS SCHIST. 273 



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varieties occur which resemble exactly in their 

 texture those varieties of graywacke of which 

 the texture seems almost purely mechanical. In 

 these, the grains of quartz appear to be merely 

 agglutinated without the intervention of a proper 

 siliceous cement, while they also bear marks of 

 previous attrition. In such specimens also it is 

 very remarkable, that, as in the analogous gray- 

 wackes, the grains in any one bed preserve an 

 average general size, whether coarse or fine; 

 and that the two are scarcely ever intermixed in 

 the same. In these also, the quartz is princi- 

 pally united by the smallest possible quantity of 

 intervening mica, which follows the minute un- 

 dulations produced by the packing of the irre- 

 gular gravel, producing an undulated and pe- 

 culiar appearance on the transverse fracture 

 which is highly characteristic. 



It is scarcely now necessary to say that the 

 minerals of which this rock is formed are, essen- 

 tially, quartz and mica. To the varying pro- 

 portions and disposition of these, are owing all 

 the leading varieties which are found in this 

 rock. As it differs from gneiss, chiefly in ex- 



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