MAPPING OF THE CRYSTALLINE SCHISTS 867 



exploited in the literature, 1 whereas, as pointed out, the consid- 

 eration of deformations by a system of fractures is, at least for 

 American areas of the crystalline schists, a new departure in 

 geological mapping. 



DIRECT GENETIC RELATIONS OF JOINT SYSTEMS TO SYSTEMS 

 OF NORMAL FAULTS. 



As early as 1879 Daubree brought out clearly the relation of 

 normal faults to planes of jointing. As in the case of so many 

 other important discoveries, however, little weight seems to 

 have been attached to the results by contemporary geologists, 

 and it is doubtful if one geologist in ten now records with care 

 the direction of joint planes. His conclusions the eminent 

 French geologist states as follows : 



1. The constancy over large areas of the orientation of certain systems 

 of joints has been already confirmed by Sedgwick, de la Beche, John 

 Phillips, and later by other geologists. Further, it has been recognized in 

 Cornwall that the joints maintain their direction in passing from the granite 

 into the schist or killas to traverse which they penetrate. 



This permanence of orientation of the joints makes them 



approach faults, of whose mechanical origin there is no doubt. 



It is further known that in many countries joints may be seen passing by 

 various intermediaries into faults properly so called. 



2. The direction of joints in Yorkshire has been the subject of a great 

 number of observations of another eminent geologist, John Phillips, 2 who has 

 collected them into a rose of directions : it results that two directions 

 predominate greatly over the others and that the two directions are 

 perpendicular to each other. 



3. [Referring to the deformation and distortion of fossils in the vicinity 

 of joints.] ..... It is therefore an important result to have arrived at a 

 consideration of joints as the effect of rupture the same as faults, which 

 differ above all by their dimensions. 



4. [Referring to the pebbles of conglomerate which have been observed 



to be neatly cut through by joint planes.] A powerful cutting or 



shearing action has operated in the formation of joints. 



To summarize, the characteristic feature which manifests itself in the 

 innumerable fissures of the earth's crust is a parallelism which reproduces 

 itself in the large and in the small fractures — in the fault as in the joint. 3 



'See Willis, loccil.; Margerie et Heim, op. cit.; Van Hise, loc. cit. 



* Phillips, Illustrations of Yorkshire, Vol. II, 1836. 



3 Daubree, Geologie Experimentale (Paris, 1879), pp. 304-6. 



