THE nOKXBLEXDlC SCUKSTS. 203 



IV. No. 16765. Dark ooloreil. linel.v Ii.tikIciI micaceous schist, from near .SW. corner of .sec. '^^>, 

 T. 48 N., R. 30 W. Much altered. Large feldspar grains, small ([uantity plagioclase, irregular (juartz, 

 little bornbleude, considerable biotite. Analyst, George Steiger. 



Structure cataclastic, approaching sedimentary in appearance. 



V. No. 16913. Light-gray banded schist, from 700 steps N., 1450 steps W., of SE. corner of sec. 3, 

 T. 47 N., R. 30 W. Not so much altered as 167G.5. Feldspar, quartz, some biotite, and a very little 

 hornblende. Kaolin quite abundant. Analyst, George Steiger. 



Structure granulated, approaching sedimentary fragmeutal. 



VI. No. 16922. Very dark gray foliated schist from 520 steps N., 1120 steps W.,of SE. corner 

 of sec. 30, T. 47 N., R. 30 W. Banded in the field, but not in hand specimen. Contains large irregular 

 quartz grains and comparatively fresh plagioclase and orthoclase. Analyst, George Steiger. 



Structure foliated, like typical crystalline schist. 



Upon compari.sdu of these analyses it will be seen that the micaceous 

 schists are in most respects as mucli like the g-ranites as they are like the 

 slate. With reference to the percentages of CaO and MgO present in 

 them, they are much more like the granites. The granitite contains three 

 times as much CaO and l\IgO, and the amphibole-granitite two and one- 

 half times as much, while the slate contains, on the other hand, over five 

 times as much MgO and CaO. The biotite-schist. No. 16922, contains twice 

 as much CaO as MgO, wdnle in Nos. 16913 and 16765 the excess of CaO 

 over MgO is probably even greater; so that if the analyses show anything 

 they indicate that the schists are altered granitites rather than altered sand- 

 stones or shales. In other words, the weight of evidence, while by no means 

 conclusive, is indicative of an igneous rather than a sedimentary origin for 

 the rocks in question, and is in accord with the little evidence afforded 

 by the microscopic investigation of their thin sections. Whether the rocks 

 were flows of -acid lava interbedded Avith the rocks that yielded the horn- 

 blendic schists, or whether they were in large part beds of tuff, has not been 

 determined. The even banding of many of the schists may be thought to 

 indicate the latter origin, but even banding is known to be characteristic of 

 some lavas, and in dynamically metamorphosed rocks, like the micaceous 

 schists, it is known sometimes to be the direct result of mashing. 



THE UORNBLENDIC SCHISTS. 



Those schists whose ])redominaut bisilicate constituent is a green horn- 

 blende may be divided into two classes, between which, however, there 



