198 THE :mai:quette ikoxbeaking district, 



lie bear the same relations to tliem as does the fine-grained fragmental 

 matrix to the larger grains in a sand}- slate. Large areas of the matrix 

 polai-ize with indefinite outlines, resendjling the irregular outlines of crys- 

 talloids of feldsjjar in a granite — a phenomenon due to the remains of 

 slightly altered feldspar left between the meshes of its alteration products. 

 The larger quartzes eni.bedded in this groundmass of decomposed feldspar 

 are all compound. The elongated grains are made up of a coarse mosaic 

 of smaller grains, the direction of whose longer axes appears to be inde- 

 pendent of that of tlie larger aggregate. Under crossed nicols these rocks 

 resemble a lot of nests of quartz mosaic in a groundmass composed of 

 ill-defined plagioclase grains and large flakes of brown biotite in a matrix 

 of small grains of quartz and fibrous decomposition products of orthoclase. 

 Tlie^- thus simulate very strongly certain sedimentary schists. 



As the (juantity of feldspar increases the superficial resemblance of the 

 schists to fragmental rocks becomes stronger, for the alteration products 

 are greater in quantity and the original outlines of the feldspathic grains 

 are more and more obscured. 



HORXBLENliIC BIOTITE-SCHISTS. 



The hornblendic micaceous schists differ from the feldspathic yarieties 

 in that they contain large crystals (tf green hornblende that are idiomor- 

 jjhic in cross-section and are frequently twinned. They foi'm yery much 

 larger plates than do any other minerals in the rocks, and often these plates 

 surround and inclose a half dozen or moi'e grains of quartz or feldspar. 

 The hornblende is evidently the latest mineral formed in the rocks in which 

 it occurs, and is quite certainly secondary. Otherwise the hornblendic 

 varieties are similar to the feld.spathic micaceous schists. 



STRUCTURE. 



The structure of the muscoyite-schists, and of many of the biotitic 

 yarieties, is that of typical schi.sts (see fig. 8, p. 197). Others of the biotite- 

 schists, particularly those containing much feldspar, have the cataclastic 

 structure, which in many cases resembles the fragmental structure of a 

 sedimentary rock. Howeyer, although their components are broken and 



