858 A TREATISE ON METAMORPHISM. 



cases they are usually flattened and more or less granulated and recrystal- 

 lized. At the same time a schistose structure develops in the matrix, and a 

 schist-conglomerate or conglomerate-schist is thus produced. (PL VIII, £.) 



Where the mass-mechanical movement is very severe the pebbles may 

 be flattened into lamina?. Since material does not readily migrate in the 

 zone of rock flowage, each pebble passes into minerals which may be 

 derived from it. If there are different kinds of pebbles, it follows that 

 there are various layers composed of different combinations, or at least 

 different proportions, of the same minerals. Large bowlders may be 

 observed in different stages, from oval forms, through greatly elongated 

 and widened fragments, to forms in which the greatest diameters are several 

 times the normal. (PL VIII, A and J5.) Such a rock, when broken par- 

 allel to the direction of greatest flattening, appears to be a perfect schist 

 or gneiss, but at right angles to this direction still plainly shows its con- 

 glomeratic character. As the process goes on the pebbles and bowlders are 

 flattened more and more, until they become thin, lenticular masses, perhaps 

 scarcely thicker than cardboard, or even like sheets of paper of great size. 

 Wherever there was a quartz pebble there is apt to be a lamina composed 

 of a quartz aggregate. Wherever there was a granite pebble there is 

 usually a lamina in which mica has abundantly developed, and therefore 

 one composed of mica, quartz, and feldspar. In many cases in which the 

 process of transformation is nearly complete one may fail altogether to 

 recognize the lamina; as pebbles in masses split parallel to the direction of 

 greatest elongation; but when a specimen is cut in a direction transverse to 

 this the pebbles distinctly appear. When the process has gone as far as this 

 the material of the pebbles becomes so mingled with that of the matrix that 

 it is difficult to exactly define the outlines of the fragments. Finally, as 

 the process continues, no evidence of the pebbles and bowlders is left. In 

 their places are thin laminse of material of a mineralogical character differ- 

 ent from that of the adjacent laminae. 



At the same time the bowlders are being transformed the matrix is also 

 being recrystallized. At various steps of the process the matrix may be, in 

 turn, slate, schist, and foliated schist. Before the pebbles are destroyed 

 the folise of the matrix wind in and out, remaining- approximately parallel 

 to the nearest pebble, which acts as a transmitter of force. In proportion 

 as the process continues, and the pebbles and bowlders approach oblitera- 



