228 C. R. VAX ills]-; — PRE-CAMBRIAN OF THE BLACK HILLS. 



parallel lines. In some cases these lines arc parallel to the schistose struc- 

 ture ; in others they are at an angle of about 60° to that structure, as though 

 the time of the growth of the mica marked a different epoch from that of 

 the garnet. Often the black material is arranged in such a fashion as to 

 show to some extent the manner of the growth of the garnets — that is, 

 included particles are arranged in lines which radiate from the centers. 

 Further, a garnet area is often subdivided by strongly marked lines of inclu- 

 sions, so that it has the appearance of consisting of several individuals which 

 have a cleavage which in each part is approximately parallel. This appar- 

 ent cleavage is of course not that, but is due to rows of inclusions. 



The staurolite, as is characteristic with this mineral, includes alike all the 

 other constituents with which it comes in contact, sometimes the area which 

 a crystal occupies being fully one-half taken by extraneous minerals. 



The tourmaline, even to a greater extent than the garnet, has shown power 

 to wholly exclude all other minerals. One tourmaline mica-schist is worthy 

 of a detailed description. This rock is a black, evenly laminated one, show- 

 ing large folia of mica and lustrous crystals of tourmaline; is strongly 

 foliated so that its cleavage surface gives back a brilliant sheen. It is finely 

 laminated, 'the lamina' being of slightly varying color. Altogether the rock 

 has the appearance of a completely crystalline hornblende-gneiss. As 

 examined in thin section the background appears to consist almost wholly of 

 quartz, although there may be more feldspar present than would bethought. 

 There is no decisive evidence that any of this quartz is fragmental, but 

 occasional grains have vague lines of inclusion in their outer parts which 

 may indicate that clastic cores are present. Scattered through this back- 

 ground in about equal quantity arc tourmaline and mica, the latter includ- 

 ing both muscovite and biotite. The tourmaline is almost wholly in well 

 defined crystals. The section is cut parallel to the schistose plane of t he 

 rock — i. '., parallel to the greater number of laminae of mica, — so that most 

 flakes are basal. The tourmaline, on the other hand, is as uniformly cut 

 parallel to the vertical axis. There is also a tendency for the majority of the 

 vertical axes i" arrange themselves in a common direction in the plane ol 

 schistosity, but this failsto carry with it many individuals. If pressure has 

 been the controlling force in the initial arrangemenl of the particles of mica 

 and tourmaline, these relations — i. e., the biotite basal and the tourmaline 

 longitudinal— are jusl what would be expected. The tourmaline is abun- 

 dantly included in nil the other minerals. The biotite frequently contains 

 black particle- of ferrite, which suggests that possibly this accessory has been 

 the factor which controlled the location of the folia of biotite. 



Immediately adjacent to the large granite masses in the southern pari of 

 the hills are found certain very <• »arse muscovite-biotite gneisses which con- 

 tain much feldspar. These, upon the whole, are more analogous to the granite- 



