THE MIOACEorS S(JL1I8TS. 



197 



of elongated (juartzes, brown biotite, |)lagi(>clas(', and ortlioclasc or its decom- 

 position products, kaolin, sericite, and epidote. Garnets are present in some 

 sections, and in others tourmaline occurs in very small (piantity. The 

 quartz and the biotite present no unusual features. 'I'lic latter mineral is 

 often chloritized, as is also some of the orthoclase, so that the (piantity of 

 chlorite is nuich greater in these rocks than it is in the nonfeldspathic schists. 



The feldspars are the most interesting constituents. They are nearly 

 always much altered, the orthoclase more so than the plagioclase. In the 

 triclinic feldspar the twinning bars are always reccignizable and the material 

 of the grains is often clear. With 

 the orthoclase the case is different. 

 Occasional traces of Carlsbad twin- 

 ning are obscurely visible, but the 

 mineral is so clouded with flakes of 

 sericite, kaolin, chlorite, and brown 

 biotite, Avitli grains of epidote and 

 quartz, needles of green hornblende^ 

 and the dust of magnetite, that its 

 original nature in most cases is dif- 

 ficult to prove. 



The arrangement of the decom- 

 position products of the orthoclase f,q 

 is irregular. Within the body of tlie 

 mineral they form a web of inter- 

 woven spicules, in the interstices of 

 which are little grains of quartz and small areas of the undecomposed 

 feldspar. Portions of the secondary aggregate extend bevond the original 

 outlines of the grains and penetrate between the primarv quartzes and the 

 biotites. Thus it frequently seems as though the rock were a fragmental 

 one, since we find rounded grains of quartz and irregular flakes of brown 

 biotite embedded in a fibrous groundmass which in appearance is not unlike 

 the material of a biotite-slate. A close inspection of the aggregate, how- 

 ever-, shows that the quartz grains have not the outlines of waterwom 

 grains, nor does the fibrous and finely granular groundmass in which they 



Thin sectinn of f.-Mspnthic biotite-s.hi.-it. No. 16903^ 

 from875,slup8X,, 12JstLlis W., of SE. comer of seen, T. 4TX., 

 R. 30 W. Section shows typical structure of the coar.ser 

 schists, rich in feldspar. The light-coloretl, irregular grains 

 are quartz, the cloutly ones feldspar, and the darl; ones bio- 

 tite. Natural lijiht X 55. 



