DISORDERLY FRACTURE FIELDS 155 



Of like tenor are the following statements by Hill:°^ 



"In this mountain two distinct systems of faulting are discernible. The first 

 system may be known as the north-south and northeast system, the other as 

 the northwest fault system. The north 80 degrees west faults are related to 

 the northwest system in age. 



"Many of the north 40 degrees west faults appear in parallel belts across the 

 mountain range and nearly all the mineral outcrops seem to be closely asso- 

 ciated with them. 



"As the writer has previously shown, nearly all of the great ore localities of 

 Mexico are associated with faults in this north 40 degrees west direction. 

 These faults are all of late geological origin, and probably the movements of 

 the earth which made them are still going on, as testified by hot springs in the 

 vicinity." 



DISORDERLY FRACTURE FIELDS 



It sliould not be inferred that the above described structural directions 

 are supposed to be the only ones which have been found within the areas 

 described. Detailed study of special districts has brought out the fact 

 that there are far more complex fracture systems on vertical planes, ar- 

 ranged for the most part in parallel and intersecting series, as, for ex- 

 ample, the Pomperaug Valley system of Connecticut (see figure 29).^* 

 The river system of the larger area of the State of Connecticut brings 

 into prominence a portion of these fracture systems, as well as some addi- 

 tional ones.^^ Wherever such definitely oriented fracture series are to be 

 made out, the control would appear to be largely exercised by disjunctive 

 planes which approach the vertical. 



Many fracture fields have, however, been studied in detail and found 

 to reveal a complex of fractures so disorderly as to have defied arrange- 

 ment within any regulated system. In the opinion of the writer, such 

 districts may be explained by the superimposition upon a simpler system 

 of later fractures due, presumably, to special and local conditions. Thus 

 in figure 30 has been represented in a schematic way what is believed to 

 be the fracture complex of central North America. In districts which 

 may be either contiguous or widely separated, fracture patterns represent- 

 ing various combinations of the elements of a common type pattern have 

 been made out, whereas in many intervening districts more disorderly 



■^3 Robert T. Hill : Geology of the Sierra Almoloya, with notes on the tectonic history 

 of the Mexican plateau. Science, new series, vol. 25, May 3, 1907, pp. 710-712. 



^ The Newark system of the Pomperaug Valley, Connecticut. Twenty-first Annual 

 Report of the U. S. Geological Survey, pt. iii, pp. 1-162, pis. 1-17, figs. 1-59. 



^' W. H. Hobbs : The river system of Connecticut. Journal of Geology, vol. 9, 1901, 

 pp. 469-485, pi. 1. 



Xll— Bull. Geol. Soc. Am., Vol. 22, 1910 



