188 A TREATISE ON METAMORPHISM. 



important factors which enter into the character of the rocks are the 

 strength and the mineral composition. The stronger the rock the greater 

 is the depth at which fiowage begins. The rocks the materials of which 

 are refractory, as, for instance, those composed of quartz and feldspar, 

 reqnire a greater depth in order that deformation may take place by flow- 

 age than those rocks the materials of which are readily acted upon 

 chemically, as, for instance, calcite. 



The higher the temperature the less the deptli in order that deforma- 

 tion may take place by fiowage. Since the temperature increases normally 

 at the rate of 1° C. for 30 meters, and since in consequence of orogenic 

 movements and igneous intrusions the increase in temperature with depth 

 is often much more rapid than this, heat is a very important factor in the 

 depth at which rock fiowage occurs. 



Since rock fiowage may be in large measure by recrystallization, and 

 recrystallization is dependent to a large extent upon the amount of water 

 present, the greater the amount of water the more readily does deformation 

 take place by fiowage and therefore the less is the depth at which fiowage 

 begins. 



The speed of deformation is also of very great consequence in limiting 

 the upper part of the zone of fiowage. The more rapid the deformation the 

 greater the depth of the zone of fiowage; the slower the deformation the 

 more moderate its depth. Speed of deformation, and therefore the time 

 consumed in a given deformation, is of very great importance. It is well 

 known that a stress not sufficient to rupture a material or to appreciably 

 deform it within a short time, if applied for a long time may produce 

 important fiowage deformation. The geologist must give this factor of 

 time greater weight than scientists in any other subject. How important 

 it is may be illustrated by the deformation of rock" as a result of placing it 

 in an unusual position. In cemeteries marble slabs have been placed 

 horizontally and supported at the ends ; in the course of a score or more of 

 years such slabs are found to have sagged in the middle a very considerable 

 amount. This is illustrated in the cemetery of Jefferson City, Mo., where 

 a slab about 1.8 meters long, .9 meter wide, and 5.08 centimeters thick, 



"Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Eept. 

 U. S. Geol. Survey, pt. 1, 1896, p. 594. 



