"Jl! 1 i. I;. VAN HISE — l'i;i:-< \mi:i; | an OF Till: BLACK HILLS. 



those in which the feldspars arc practically unchanged or surrounded by a 

 mere film of biotite and muscovite to those in which, in place of a lai 



■ i 11 of feldspar, is found a thoroughly interlocking mass ol muscovite, 

 biotite, and quartz. 



One rock presents a modification of this process which is worthy of note. 

 Macroscopically it contain- a -nod many roundish or oval fragments of black, 

 aphauitic, cherty-looking material, some of them one-fourth nf an inch or 

 more in diameter. Under the microscope these turn out to lie feldspars 

 which have been cracked and impregnated through and through with the 

 ferrite found bo plentifully in many of the rocks. Their true nature is dis- 

 coverable only in those cases in which the amount of this material is smaller 

 than usual. Gradations are found from grains of which the character is 

 evident to those almost opaque from included ferrite. 



The original quartz grains have generally heen elongated in a direction 

 parallel to the schistose structure, as in the conglomerates and quartzites 

 before described. In many cases ii is not possible at the present time to tell 

 what part of the quartz is original and what secondary; but frequently, 

 simultaneously with the other changes, has heen deposited abundant ferrite, 

 just as in the quartzites and conglomerates. When this bas occurred it 

 murk- off with perfect clearness the original fragmental quartz from the 

 ondary minerals. When the schists have become more thoroughly crys- 

 talline the only minerals now present which were originally deposited as 

 such are the cores of quartz in the larger elongated particles. In certain 

 cases the fragmental character of the quartz grains is not shown by such 

 inclusions but by minute Hakes of white mica, which are included in the 

 enlargements and lie in curved lines about the Cores. These are not BO con- 

 tinuous as the ferrite inclusions, bul arc sufficiently so to form well defined 

 oval-. Unlike the former, these folia are only discovered in polarized light. 



( >fteii the fragmental quartz which has heen mingled with the feldspar has 

 been rather fine-grained. In these cases it is not at a glance distin- 

 guishable from newly developed quartz. In other cases the quartz particles 

 have been large; and here, unless the pressure has heen very great, they 



ml out with rounded outlines in a thoroughly crystalline matrix (figs. I 



and ■'. plate 1), However, in the most crystalline phases of the schists 

 immediately adjacent to the granite, the pressure has been so great that even 

 when the fragmental quartz was coarse the rock has now an evenly 

 miliar, roughly banded arrangement of mica and quartz. These rocks 

 are as c larselj and completely crystalline as mica schists which occur in the 



indisputably fundamental gneiss The quartz and mica are concent rated 



more or less iii alternate hand- and irregular area- just as in such rocks. 

 The mica folia average about 1'" '" in greatest length, and the quartz pai 

 tides, of quite uniform size, arc one-half i le-fourth as long. The only 



