STAGES IX THE METAMORPHISM OF THE MICA-SCHISTS. 225 



thing which now shows the original position of the clastic particles of quartz 

 and feldspar is the relative distribution of the minerals. The areas in which 

 quartz is almost the sole constituent probably represent quartzose fragments 

 which have been broken down by dynamic action, while the areas which are 

 largely micaceous probably represent places once occupied by feldspar. 



As we pass from the less crystalline to the more crystalline mica-schists 

 there is a gradual increase in the size of the secondary quartz particles. This 

 is just what would be expected from their manner of development. The 

 more plainly fragmental the rocks are, the finer crystalline is the back- 

 ground. Naturally when the recrystallizing forces have become greater the 

 particles which are autbigenic grow to a greater size, and this process being 

 accompanied by powerful dynamic action the large fragmental quartzes are 

 at the same time broken into small particles (fig. 2, plate 4). It follows 

 that it is entirely possible to produce from a coarse-grained quartz-feldspar 

 detritus a crystalline schist in which the quartzose background is composed 

 of grains of approximately uniform size, and which contains mica in large 

 flakes, scattered here and there in bands or irregular areas. That this state- 

 ment represents the actual facts in certain schists of the Black Hills, except 

 that the broken down quartz is a little coarser than the authigenic, cannot be 

 doubted by any one, I think, who will observe the gradual transitions in the 

 field and see the corresponding mineralogical changes in thin section. 



In the mica-schists the two micas, muscovite and biotite, are both abun- 

 dant, although biotite is upon the whole rather more plentiful. Occasionally 

 muscovite is predominant. Frequently also chlorite in well defined leaflets 

 is present as a subordinate mineral. These minerals are for the most part 

 secondary developments. If any original mica is now present, it is in sub- 

 ordinate quantity. The micas are arranged to a remarkable degree with 

 their longer axes in a common direction parallel to the schistose structure 

 (fig. 2, plate 5). Sometimes, as will be seen, w T here there is a slaty cleavage 

 or schistose structure in two directions, the mica flakes show a peculiar 

 double arrangement corresponding to them (fig. 1, plate 5). The general 

 perfection of the linear parallel arrangement of the micas and the quartz, 

 the beauty of the former minerals, and the absence of all others as impor- 

 tant constituents combine to make these rocks the most perfect examples of 

 mica-schists that I have seen. 



The greywackes, mica slates and mica-schists frequently become very 

 fine-grained and pass into aphanitic slates and schists. These, however, 

 need no detailed description, as they repeat with smaller particles the same 

 story told by the coarser-grained rocks. In certain of them, evidence of 

 fragmental origin is found ; in others it is wanting. These rocks appear to 

 have differed chiefly from the mica-slates and mica-schists in that the original 

 detritus was much more finely comminuted and doubtless contained a rela- 



