736 A TREATISE ON METAMORPHISM. 



recrystallized as a white, silky bucholzite, or fazer-kiesel in the granite, and the gra- 

 phite scales are inclosed in all the constituents of the granite over many square miles. 



Over the whole surface of the great Hubbardston batholith of perfect coarse 

 porphyritic granite, 51 kilometers long and 10 kilometers wide, it was possible to 

 map the areas once occupied by the different schists, which formerly mantled over 

 the granite mass, by means of the indestructible constituents of the former schists, 

 by the portions which had melted into the mass of the granite, by the filaments still 

 remaining unabsorbed, and by the different aspect of the granite, dependent largely 

 upon the great increment of iron. Using the first two criteria especially, this double 

 mapping of the region was carried out in the sheets prepared for the United States 

 Geological Survey. 



The region extending for several kilometers south from Mount Wachusett, and 

 that north and south of Brookfield, in the Hubbardston batholith, furnish abundant 

 exposures for the study of the phenomena here described/' 



The statement of facts which Emerson makes seems clearly to lead to 

 the conclusion that the granites of the Worcester district have actually- 

 fused the metamorphosed schists to some extent. However, the changes 

 in chemical and mineral character of the granites adjacent to the schists 

 may be partly due to the transfer of material from the injected rock by 

 solutions. Until the facts for this district are given in detail, including 

 chemical analyses of the different phases of the granites, it will be impos- 

 sible to say how far the intruded schists have been absorbed by the granite; 

 and certainly the assumption that the differences between the Worcester 

 granites and the Quincy granites are due to absorption of schistose material 

 by the former requires much more evidence than has yet been offered. 



COMBINATIONS AND RELATIONS OF THE VARIOUS PROCESSES. 



The processes of welding, strain within the elastic limit, strain beyond 

 the elastic limit, cementation, metasomatism, and injection rarely occur 

 separately. Commonly several of them take place at the same time, and in 

 many instances all of them unite in the metamorpliism of a rock. In the 

 previous pages, for the sake of clearness in analysis, each process has been 

 considered by itself, so far as this was possible, so that the effect of each 

 could be clearly appreciated. This natural tendency to consider each 

 process separately, and the tendency to regard a certain process under 

 consideration as the sole cause of metamorphism, have led to the artificial 

 classification into dynamic metamorphism, contact metamorphism, etc 



« Emerson, B. K., Difference in batholithic granites according to depth of erosion: Bull. Geol. Soc. 

 America, vol. 10, 1899, pp. 499-500. 



