LITHOLOGY OF THE SERIES 865 



tite. The hornblende is in strongly pleochroic plates, with prismatic 

 habit and generally parallel position. Figure 3 is from a representative 

 hand specimen. Magnetite is present. Frequently its sections are quad- 

 ratic, and they are strewn through every other mineral part of the sec- 

 tion. Garnets show the usual tendency toward absorbing quartz in their 

 growth to a considerable proportion of their bulk. Other minerals are 

 rare. Staurolite appears farther westward as an important accessory. 



THE HORNBLENDE-BIOTITE SCHISTS 



These rocks differ in only a slight degree from the hornblende-schists 

 just discussed. They carry biotite in addition to the minerals named 

 in the preceding rock, are quite fresh looking in normal phases, and their 

 texture is so fine that it is difficult to distinguish them from those carry- 

 ing only hornblende as the bisilicate constituent. As a rule, the horn- 

 blende folia are very minute compared with the biotite. There are but 

 few accessory minerals, garnet being the most conspicuous, while, mi- 

 croscopically, magnetite appears. 



These rocks have every appearance of having resulted from the altera- 

 tion of the hornblende schists. It must be admitted, however, that the 

 evidence that such alteration actually occurred is incomplete. Speci- 

 mens were not secured in sufficient quantity and from crucial points to 

 establish the series or demonstrate the genetic relationship. In inter- 

 mediate phases traces of the earlier constituents can be seen. Veins are 

 frequent in these rocks. Their contents are made up partly of enlarge- 

 ments of the neighboring broken mineral grains, and thus point to the 

 alteration processes as in progress. 



THE BIOTITE SCHISTS 



These rocks occur in several localities. In general field characters 

 they are indistinguishable from the partially altered graywackes or the 

 hornblende graywackes already described. The freshest phases show 

 hornblende in small proportion ; normall}'' they carry biotite as the al- 

 most exclusive bisilicate constituent. Garnets are a characteristic fea- 

 ture of the entire series of rocks which have resulted through the com- 

 plete alteration from the graywackes. In the biotite schists the garnets 

 are altering into serpentinous products ; some crystals having entirely 

 disappeared, the resultant product, as a finely crystalline mass, presents 

 the optical reactions of serpentine. 



THE STA UROLITIC BIOTITE SCHISTS 



These biotite-bearing rocks are characteristic along the Mississippi 

 river for about 10 miles north and south of Little Falls. The strike is 



