ALGONKIAIvT EOCKS OF STUEGEOF EIVEE TOI^GUE. 477 



PETROGRAPHICAL DESCRIPTIONS. 



As might naturally be expected, the least schistose of the arkoSes and 

 conglomerates exhibit the fewest evidences of alteration in the thin section. 

 In addition to the pebbles in the conglomerates, these rocks consist of 

 rounded and angular grains of quartz, microcline, orthoclase, and of various 

 plagioclases, and a few of microperthite, embedded in a finer-grained 

 aggregate of the same minerals, tiny flakes of green biotite and of color- 

 less muscovite or sericite, a few plates of chlorite, particles and crystals of 

 magnetite, and little nests and isolated grains of epidote, with occasionally 

 some calcite. Many of the feldspar grains are altered into sericitic products, 

 colored red by small particles of various iron oxides and by red earthy 

 substances. 



The composition and microstructure of the schistose arkoses and of 

 the schistose matrices of the conglomerates vary greatly in different speci- 

 mens, being determined largely by the original composition of the different 

 beds and the amount of squeezing to which they have been subjected. No 

 attempt will be made here to describe in detail all the changes suffered 

 by these rocks; a simple statement of the tendency of these changes will 

 be given. 



The quartz pebbles in the moderately schistose conglomerates show 

 plainly that they have been under great stresses. The smaller ones all 

 exhibit undulatory extinction. The larger ones are sometimes peripherally 

 granulated, and sometimes etched or corroded on their edges, as though 

 they had suffered partial solution. By this process small portions of the 

 original particles have been separated from them, and the dissolved silica 

 has been redeposited among the grains of the surrounding matrix as sec- 

 ondary quartz. In their interiors many of the larger pebbles havf) been 

 changed to a mosaic of differently oriented parts, which interlock so per- 

 fectly that they appear to have crystallized together. 



The groundmass in which the pebbles lie is, in a few cases, a frag- 

 mental aggregate of quartz and several feldspars, with the addition of seri- 

 cite and other crystallized components. In most cases, and in all in which 

 schistosity is marked, no fragmental structure is noticeable. The ground- 

 mass is an interlocking mosaic of fairly large quartz grains that appear to 

 have crystallized in situ, between which are smaller grains of the same 



