GEOLOGY 555 



The densities were computed from the norms of the average 

 igneous rocks of the various regions. For the United States 

 Washington demonstrates a remarkable correspondence between 

 comagmatic regions and areas of positive or negative gravity 

 anomahes. Since Washington is deahng with compact, almost 

 pore-free, igneous rocks, the considerations emphasised by 

 Lane do not apply. 



In his Presidential Address to the Geological Society Dr. 

 R. D. Oldham {Quart. Jonrn. Geol. Soc, 78, pt. i, 1922, pp. 

 Iv-lxviii) deals with the cause and character of earthquakes. 

 He shows that the rate of growth of strain which produces an 

 earthquake may not be so slow as sometimes imagined, but 

 may take place in a period of the order of a few months. The 

 faults with which earthquakes are usually associated are not 

 the " causes " of the earthquakes, but merely serve to localise 

 and control the distribution and intensity of the seismic pheno- 

 mena. In many cases tectonic processes cannot be the cau?e 

 of the strain, and Dr. Oldham suggests that rapid changes 

 of volume in the sub-crust, due to sudden changes in modes 

 of mineral aggregation, may be the cause of many earth- 

 quakes. For the production of bursting pressures in magmas 

 during crystallisation see Science Progress, January 1923, 

 p. 362. 



S. Taber's investigation of the great fault troughs of the 

 Antilles {Journ. Geol., 30, 1922, pp. 89-114) suggests that the 

 east to west arcs delineating the major relief features of this 

 region are zones of normal faulting developed in late geological 

 times ; and that this faulting has resulted in deep narrow 

 troughs, mostly submerged. These resemble rift valleys with 

 precipitous enclosing scarps, and floors which, instead of being 

 graded like river valleys, rise and fall throughout their lengths. 

 Earthquake phenomena indicate that the displacements are 

 still continuing, but Taber finds no causal connection between 

 the relatively meagre volcanic activity of the Antilles and the 

 foundering of the narrow blocks. 



Recent German literature on similar topics includes the 

 following : 



L. KoBER, Dey Bau der Erie, Berlin : Gebruder Borntraeger, pp. 324. 



M. Weber, " Zum Problem der Grabenbildung," Zeits. d. D. Geo!. Ges. Abh., 



73, 1921, pp. 238-89. 

 H. Quiring, " Eiszeit und Gebirgsbildung," Zeits. d. D. Geol. Ges. Monatsber., 



1921, pp. 281-8. 



E. NowAK, " Sedimentation und Gebirgsbildung," Naturwiss., 44, 1921, 



pp. 892-7. 

 H. Quiring, " Gebirgsbau der Ostkarpathen, Deckenlehre und Vulkanismus," 



Zeits. d. D. Geol. Ges. Monatsber., 1921, pp. 108-29. 



