6o2 C. K. LEITH AND W. J. MEAD 



minerals, and 12 per cent quartz. The schists or gneisses de- 

 veloped from these igneous rocks have a considerably smaller 

 proportion of feldspars. The ferromagnesian minerals, instead of 

 consisting dominantly of augite and biotite, with some muscovite, 

 are now largely hornblende and muscovite, with subordinate 

 quantities of biotite. 



Whether the parent rock is igneous or sedimentary, or what- 

 ever its mineral content, the resulting schists and gneisses are 

 characterized by hornblende, chlorite, and mica, which are devel- 

 oped to such an extent that the very nature of the original rock is 

 often lost. If the mineralogical change were not extensive, the 

 problem of origin of schists and gneisses would not be nearly so 

 difficult as it is. The very existence of the problem testifies to the 

 great mineralogical changes which have occurred. A sericite 

 schist, for instance, may result from d3niamic anamorphism of a 

 slate, an igneous rock, or a quartzite. A hornblende schist may 

 result from the anamorphism of a limestone or marble or may 

 develop from a basic igneous rock, a graywacke or other basic 

 sediment, or a slate. 



A few distinctive minerals, such as mica, chlorite, and horn- 

 blende, are the characteristic resulting products of anamorphism 

 by rock flowage of a considerable variety of parent rocks. In 

 this sense, then, there is a convergence toward a certain mineral- 

 ogic type. 



Chemical changes during anamorphism by rock flowage. — 

 When the chemical changes in anamorphism by rock flowage are 

 considered, there is less general acceptance of the fact that actual 

 changes in composition take place and that these changes are in the 

 direction of producing the composition of the characteristic minerals 

 in the end-products. Probably it is the general view that the 

 conditions of rock flowage and anamorphism are not usually favor- 

 able to extensive transfers of materials involved in change of com- 

 position. It is often true that there are no important changes in 

 percentages of chemical constituents during the process of rock 

 flowage and anamorphism, the mineralogical change having been 

 accomplished through recrystallization of substances present. 

 This is especially true when the original rocks are well adapted, in 



