RESEMBLANCE OF ROCKS TO THOSE OF LAKE SUPERIOR. 229 



than to the mica-schists and mica-gneisses above described. Their genesis 

 is uncertain. They may be merely foliated phases of the granite, or they 

 may be masses of the clastic rocks which were caught within the granite and 

 so profoundly altered as to lose all trace of their fragmental character.* 



Other Crystalline Rocks. — I shall not attempt in this paper to give a 

 detailed description of the several other kinds of rock which occur in the 

 pre-Cambrian core of the Black Hills. In order to make a comprehensive 

 comparison with other localities, it is, however, necessary to briefly charac- 

 terize them. 



The first in importance among these are the quartz-rocks, ferruginous 

 cherts and schists. The chief constituent of this class is finely but com- 

 pletely crystalline interlocking quartz in particles of quite uniform size. No 

 evidence is observed that any of them are fragmental. The chief remaining 

 substance is iron oxide, which occurs in the forms of limonite, hematite, and 

 magnetite. The only other constituent of importance is muscovite. Occa- 

 sionally a little biotite and iron carbonate are seen. The iron oxide is often 

 concentrated into layers, and thus locally makes up a considerable propor- 

 tion of the rock. The highly ferriferous layersare interlaminated with those 

 that contain much less iron oxide. These oxides usually have crystal out- 

 lines, and the particles are arranged in entire independence of the quartz. 

 The relations of the two minerals are just what they would be if the iron 

 oxide had wholly crystallized before the silica appeared. The quart/ 

 phases are called " quartz-rock " inorderto separate them from the quartzites, 

 the latter term being restrieteil to rocks which are chiefly composed of worn 

 fragments of quartz. This distinction has proved to be a fundamental one 

 in the Lake Superior region, in which the cherts and jaspers of the iron 

 formation are always non-fragmental, while the quartzites are as plainly 

 clastic This class of rock, as observed by Newton, is then remarkably like 

 in mineral character to the ferruginous schists of Lake Superior, the chief 

 difference being that muscovite in one phase of the Black Hills rock is sub- 

 stituted for actinolite in the corresponding rock of Lake Superior. This 

 microscopic likeness is no stronger than the macroscopic resemblance. 



The fact that associated with the mica-slates and mica-schists are ridges ami 

 large masses of coarsely crystalline, dark gray or green, massive to schistose 

 rocks which resemble altered greenstoues has already been mentioned. 



* In connection with the foregoing upon the development of the mica slates and schists, ihe re- 

 markable studies of Sorby, began many years ago, should be mentioned. As early as 1863 his 

 microscopic studies showed that certain mica-slates are of fragmental origin. Many years later 

 t ls.so) he again took up the subject and presented additional evidence, not only that this is true, but 

 that the mica and much of the quartz in certain rather crystalline mica-schists and slates are 

 secondary developments. He was not able, however, with the material at his hand to work out the 

 manner of the genesis of these minerals, nor does theirsonrce seem to have occurred to him except 

 in a general way as developing from the original mud. His work in connection with this study as 

 to " stratification foliation " and " cleavage foliation" is too well known to need reference. See 

 "On the ( iriginal Nature and Subsequent Alteration of Mica-Schist," II. C. Sorby, Quart. Jour. Geol. 

 Vol. XIX, 1863, pp. 401-406; "On the Structure and Origin of Non-Calcareous Stratified Rocks," 

 H. C. Sorby (part of Anniversary Address of the President of the Geological Society of London), 

 c^uart. Jour. Geol. Soc, Vol. XXXVI, 18S0, Proceedings, pp. 46-92. 



