864 STUDIES FOR STUDENTS 



blocks. If the direction of the new compression made a large 

 angle with the direction of the first, the directions of the new 

 planes of dislocation would be difficult to determine, and torsion 

 would be at a maximum. If, however, its direction corresponded 

 closely to the first, as is most likely if it occurred in the same 

 general period of disturbances affecting the area, the later planes 

 of dislocation would in their directions bear a simple relation to 

 the earlier. New depression of the area would quite likely fol- 

 low the new dislocation. 



Gilbert has shown that within an area which is undergoing 

 depression along fault planes it is likely that earthquakes will 

 follow one another in succession, owing to the fact that oro- 

 graphic blocks are supported in part by the friction along their 

 vertical walls, and that starting friction is greater than sliding 

 friction. 1 The momentum acquired by the slipping block is 

 effective in reducing temporarily the energy of the system, and 

 time is required for its recovery. 



To summarize the mechanics of crustal deformation by rup- 

 ture, it may be assumed that its result would probably be to 

 produce vertical polygonal prismatic blocks bounded by joint 

 and fault planes in a number (large or small) of parallel series 

 intersecting one another, and that these blocks would stand at 

 relatively different altitudes. 



SYSTEMS OF FAULTS. 



Continental European geologists have quite generally recog- 

 nized the importance of systems of normal faults in bringing 

 about important deformations, and have described such systems in 

 many widely separated areas. This may, in fact, be said to consti- 

 tute the most marked line of cleavage between the structural 

 geological investigations of Europe and of America. The trend 

 of European investigation in this regard was early given by 

 Leopold v. Buch in Germany and Elie de Beaumont in France. 

 The latter correctly located along the Rhine valley 2 parallel 



"Gilbert, "A Theory of the Earthquakes of the Great Basin, with a Practical 

 Illustration," Am.Journ. Sci. (3), Vol. XXVII, 1884, pp. 49-53. 



2 Dufrenoy EtElie DE Beaumont, Explication de la carte geologique dela France, 

 Vol. I, p. 398. 



