694 A TREATISE ON METAMORPHISM. 



from such sediments these minerals may be altogether absent, their places 

 being largely taken by the micas and other minerals. It is clear that 

 during the metamorphism of the rocks these minerals are taken into solu- 

 tion, and from such solutions the new minerals, containing little water, are 

 deposited. The solution and deposition give the material a less hydrated 

 and more compact form. During the process, at numerous places mica 

 nuclei oriented by the "differential stresses (see. pp. 671-673) begin to form. 

 The minute nuclei, once formed, serve as cores upon which the material 

 which is continuously taken into solution may be deposited. The mineral 

 particles grow somewhat uniformly, being subject to the same laws in this 

 respect as original particles. (See pp. 120-123.) By studying a series of 

 thin sections from any of the districts in which the rocks of a formation 

 vary from little altered material to coarse schists, all stages of the process 

 may be seen, from that in which the original hydrated minerals are abundant 

 and mica is absent to that in which the former are absent and mica is 

 abundant. 



In the foregoing we apparently have the explanation of the large 

 average size of the mineral particles which constitute the schists formed at 

 considerable depth during mass-mechanical action. They are continuous 

 growths during deformation by solution and redeposition. 



As excellent illustrations of rocks showing all or many stages of recrys- 

 tallization of quartz and the development of mica may be cited the schists 

 and gneisses which I have described in the Penokee-Gogebic and Marquette 

 districts of Michigan and in the Black Hills of South Dakota." (PL III, 

 A, D; PL XI, B, C.) 



As a beautiful illustration of the transition from finely crystalline to 

 coarsely crystalline rocks may be cited the iron-bearing formation of the 

 Marquette district of Michigan.' The deformation of this formation was 

 mainly by recrystallization. In the eastern part of the district granulation 

 and widely spaced fractures occurred to some extent, but the temperature 

 was not high enough for recrystallization, or else some other essential .con- 



« Irving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin: 

 Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 305-345. Van Hise, C. R., and Bayley, W. S., The Mar- 

 quette iron-bearing district of Michigan: Mon. U. S. Geol. Survey, vol. 28, 1897, 444-459. Van Hise, 

 C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 1890, pp. 222-229. 



i Van Hise and Bayley, cit., Mon. 28, pp. 336-375. 



