294 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
Those who have been working among the ancient and therefore more 
modified rocks are inclined to explain similar phenomena as the result 
of alteration. In each individual case the phenomena must be studied 
in the field and in the laboratory, taking into account all the evidence; 
in order to ascertain the actual truth; for it is certain that such phe- 
nomena as amphibole surrounding pyroxene cores and pegmatitic text- 
ures may be due to primary crystallization or to secondary alteration, 
and the appearance in the two cases be much the same, if not identical. 
Mass DYNAMIC ACTION AND ACCOMPANYING MOLECULAR DYNAMIC ACTION 
It has already been stated that in connection with mass dynamic 
action, molecular dynamic action invariably occurs. The kind and 
amount of resultant metamorphism varies greatly, depending upon 
depth, upon the particular kind of deformation, and upon other factors. 
It has been shown in another place that, depending upon depth, there 
are three important zones of deformation of which we have definite 
knowledge: (1) An upper zone of fracture, (2) an intermediate zone of 
fracture and flowage, and (8) a lower zone of flowage.* 
ZONE OF FRACTURE 
In the zone of fracture deformation is accomplished by considerable 
movements along surfaces or zones, with little or no movements between 
these planes or zones. Such fractures are faults, joints, fissility, bed- 
ding partings, and the spaces of autoclastic rocks. The rocks are broken 
by these fractures into great regular masses, blocks, or leaves or into the 
irregular fragments of a dynamic breccia. Into these openings water 
readily enters to assist in the modifications. The movements between 
the individual mineral particles are largely confined to thin layers along 
the walls of the openings, and the conditions may be here those of im- 
portant interior deformation, but for the masses of rock between the 
fractures the conditions are those of molecular dynamic action already 
described, and the changes are correspondingly slow. The rapid 
changes are confined to the material adjacent to the openings: From 
the places of entrance waters may permeate the adjacent rocks to a 
greater or less distance, and consequently molecular dynamic metamor- 
phism may occur to a much greater extent than it would were it not for 
the fracturing. ‘The alterations of the thin layers of material adjacent 
to the openings are by interior movements, which are in all respects like 
those of kneading described under zone of flowage. 
* Principles of North American pre-Cambrian geology, by C. R. Van Hise: Sixteenth Ann. Rept. 
U.S. Geol. Survey, part I, 1896, pp. 589-595. 
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