DEFORMATION OF ROCKS 213 
are interstratified with beds of grit or sandstone. The first may 
pass to its new position by homogeneous flow, the second by 
repeated fractures, which in extreme cases may break the harder 
beds into fragments and bury them in the softer rocks. The 
same relations often are seen between mobile marble and brittle 
gneiss. Whether fracturing always implies at least temporary 
crevices is an undetermined point. 
The weight of the superincumbent material may have been 
so great that the rock beds as a whole bent without macroscopic 
fracture, and yet the microscope may show that the individual 
grains were broken and that minute crevices were formed which 
have been subsequently filled by secondary infiltrations. At 
many places the massive beds the quartzites of Doe River, Ten- 
nessee, are bent upon themselves within their own radius, with 
no macroscopic evidence of crevice or fracture. But the micro- 
scope shows that the fracturing and resultant flattening of the 
quartz grains was almost universal. 
This illustration shows that fora given kind of rock the zone 
of fracture passes gradually into the zone of flowage. Even 
where so deeply buried that all large fractures are absent and 
the rocks are practically in the zone of flowage, the microscope 
may still show crevices. It has already been indicated that 
the zone of flowage is much deeper for some rocks than for 
others. Also for the same rock mass it may be less deep 
when gently folded than when closely folded. It is therefore 
clear that there are gradations between the three zones—of 
fracturing, of fracturing and flowage, and of flowage. In the 
placing of a rock mass in one of the three zones it is to be 
considered as belonging to the one to which it most closely 
corresponds. If the rocks are everywhere broken and show 
comparatively little folding, they are in the zone of fracture; 
if they show much fracturing and also are folded, they are in 
the zone of fracture and flowage; if the fractures are subordi- 
nate or microscopic, they are in the zone of flowage. 
C. R. VANHISE. 
