MAPPING OF THE CRYSTALLINE SCHISTS 863 



opinion of the writer, these considerations render invalid many 

 of the data collected from test pieces in laboratories. Elsewhere 

 the writer has developed this subject more extensively. 1 It has 

 there been pointed out that the production of a system of verti- 

 cal and perpendicular joint planes may be, and probably would 

 be, but the first step in the process of relieving by rupture the 

 stresses within a crustal block. Bv the formation of prismatic 

 joints the potential energy of the system is lowered and a read- 

 justment of the stresses brought about. Furthermore, a tend- 

 ency is set up favoring local depression, for the reason that rocks 

 in the zone of fracture support their load by their rigidity, and 

 this element of rigidity has been much reduced through the sys- 

 tem of dislocations. The crustal block rests upon the rocks in 

 the zone of flow, which are in a condition of potential fluidity, 

 or potential viscosity. The differences of composition and den- 

 sity within the zone of flow, combined with the varying perfec- 

 tion of the joint planes within the zone of fracture (which may 

 be due to lack of homogeneity and perhaps to other causes), 

 furnish conditions for a most irregular and unequal depression 

 of the component blocks produced by the dislocation. On the 

 basis of experiments and observation alike it is believed that the 

 type of tectonic structure produced, before planed down by 

 erosive agencies, may be well illustrated by that of a mosaic, 

 the backing of which has been removed and the component 

 blocks so disturbed as to stand at different altitudes, but with 

 similar axes parallel. 



A resumption, or, what is more likely, a continuation, of 

 strong compression of an area in the same horizontal plane, or a 

 second shock communicated to it, would doubtless produce a 

 result of a somewhat different character from the first. The 

 resultant of the lateral compressive stress is by reason of the 

 ready-formed joint-planes resolved in directions parallel and 

 perpendicular to them, the former producing a shear along the 

 planes themselves, and the latter a shear along the diagonals of 

 the blocks of the primary system. Torsional stresses are also 

 set up so as to produce fracturing at the edges of the prismatic 



1 Twenty-first Annual Report U. S. Geo!. Survey, Part III, pp. 124-33. 



