362 F. H. Lahee — Metamorphism and Geological Structure. 



ited by specimens of Stage C. Here the sericite occurs, not 

 only in the matrix and plastered outside the pebbles, but also, 

 to a certain extent, within them. Metacrysts of ilmenite, 

 biotite, and garnet are sometimes present in the matrix. Many 

 of the pebbles — even the more resistant ones — particularly 

 when examined with the microscope, show signs of distortion 

 in their own oval shape and also in the elongate form of their 

 individual grains. If two pebbles are in contact, one often 

 indents the other. The lengths of the pebbles and of their 

 constituents are roughly parallel to a secondary cleavage 

 (schistosity) which has been produced in the matrix at this stage. 



In Stage D the hardest pebbles may be sheared, flattened or 

 elongated, bent, warped, and fluted. The less resistant ones 

 have been squeezed into mere sheets and are cleaved parallel to 

 their flatness. Recrystallization in pebbles and matrix is at a 

 maximum. In both parts of the rock sericite is plentiful, and 

 the metacrysts themselves may be nearly as abundant in the 

 pebbles as in the matrix. 



The study of these phases in the psephites brought out the 

 fact that spindle-shaped pebbles (linear schistosity) are charac- 

 teristic of lower metaphorism than are flattened, sheet-like 

 pebbles (plane schistosity). Moreover, when the matrix, as a 

 whole, is more resistant to deformation than the separate peb- 

 bles, the pebbles first reveal evidences of incipient metamorph- 

 ism, and, at any given stage in the history of the rock, they 

 are at a stage of metamorphism somewhat higher than that 

 of the surrounding matrix ; and when the matrix is less resist- 

 ant than the pebbles, the reverse of this statement is true. 

 That is to say, deformation first affects the weaker portions o£ 

 the rock. 



Summary. * — The Carboniferous sediments, originally nor- 

 mal fresh- water elastics, which represented the products of 

 immature weathering, have been altered by dynamic and static 

 metamorphism. During the process certain new minerals 

 developed. In different specimens these minerals vary in 

 species, in quantity, in orientation, and in size of the individual 

 particles, and it is found that the variations are consistent with, 

 and may therefore be used to some extent as indices of, the 

 degree of metamorphism to which the rock has been subjected. 

 Other factors, such as perfection of cleavage, deformation of 

 pebbles, etc., serve in the same capacity. According to these 

 indices the specimens have been classified in groups, each of 

 which is characterized by a particular stage of metamorphism. 



*In the foregoing outline the major portion of the petrographic investiga- 

 tion, with all its details of megascopic and microscopic description, has 

 been omitted. We have endeavored to present a fair, though concise, idea 

 of the kind and amount of metamorphism, and of the method adopted in the 

 work. 



