206 Van Hise and HosTcins — Pre- Cambrian Geology. 



able for the study of pre- Cambrian rocks, it is necessary to 

 develop methods of geological work based upon physical phe- 

 nomena. The subjects considered under physical geology are 

 the general movements of rock material under deformation, 

 folds, cleavage and fissility, joints, faults, autoclastic rocks, 

 metamorphism of rocks, and stratigraphy. 



Deformation of Rocks. — It is shown that the outer part of 

 the earth may be divided into three zones : (1) An upper zone 

 of fracture ; (2) a middle zone of combined fracture and plas- 

 ticity ; (3) a lower zone of plasticity. 



1. Rocks under less weight than their ultimate strength 

 when rapidly deformed are in the zone of fracture. That is, 

 when rocks under such conditions are deformed they break, 

 and crevices small or great separate the broken parts. The 

 fractured rocks may be jointed, faulted, or brecciated in a sim- 

 ple or a complex manner. The fractures may be far apart and 

 of great size and extent, or near together and of small size and 

 extent. Innumerable parallel fractures may occur in the same 

 direction, when, as shown later, the rocks develop a parting, or 

 fissility. In extreme cases of fracturing the rocks become auto- 

 clastic, or are broken into innumerable fragments by the forces 

 of deformation. These fragments may be rounded, and such 

 rocks resemble ordinary clastic rocks. The lower limit of this 

 zone of fracture is at different depths for different rocks, and 

 at different depths for the same rock under different condi- 

 tions of deformation. However, from computations by Prof. 

 L. M. Hoskins it is concluded that it is highly probable that at 

 a depth of 10,000 meters, not only do no crevices permanently 

 exist in the earth, but the rocks are in such a condition that 

 actual welding of the fractured parts would soon take place, 

 supposing fracture to occur. 



3. Rocks buried to such a depth that the weight of the super- 

 incumbent strata exceeds their ultimate strength are in the 

 zone of plasticity and flowage. These are the conditions of 

 folding, for permanent perfect flexure is possible only by flow- 

 age of material. It is a contradiction to suppose that cracks 

 and crevices can form uuder these conditions. Were it pos- 

 sible for an opening to be made in any way, under the hypoth- 

 esis, the rock would flow toward the opening and close it. This 

 is the deep-seated zone of perfect folding. In the folding 

 there is necessarily readjustment between the strata and re- 

 arrangement within each of the strata. If the beds are not 

 changed in thickness, the folds must die out with increased 

 depth. If they remain of similar form, this is only possible 

 by a thickening on the anticlines and synclines and a thinning 

 on the limbs. 



2. Since the boundary between the zone of fracture and the 



