STUDIES FOR STUDENTS 183 



Overload glides: On land, gravitational movements occur on 

 slopes due chiefly to erosion; on the sea-floor unstable equilibrium 

 must often result from aggradation. Sediments are unequally 

 spread owing to set of current and distance from sources of supply. 

 Deltas and banks are thus built up, until along their edges overload 

 gives rise to facial shear and glide. 



Earthquake glides: The chief geologic effect of earthquakes 

 on land is to precipitate movements both of the waste mantle and 

 of the solid rock beneath. Alluvium on valley floors lurches toward 

 the thalweg, the waste on hillsides, slumps and avalanches, and even 

 solid rock may be intimately shattered and shaken down in land- 

 slides of the first magnitude. 1 The effects of earthquakes on 

 marine deposits must be of similar nature and proportionally great. 

 Evidence collected by Milne 2 proves conclusively the fact of sub- 

 aqueous glides and their close connection in a number of instances 

 with earthquakes. Since the continental delta throughout geologic 

 time has been the zone, not only of sedimentation, but also of 

 great diastrophic movements of which earthquakes are an expres- 

 sion, it may be assumed that earthquakes have been a not uncom- 

 mon cause of glides in geologic history. Yet no instance is known 

 to the writer in which a glide breccia has been assigned to this 

 precipitory cause. The most direct evidence pointing to such an 

 origin is to be found in contemporaneous associated faults or sand- 

 stone dikes. Since earthquakes recur in the same area for long 

 periods of time, earthquake-glide breccias may recur at successive 

 horizons in a formation on in a sequence of formations. 



Deformation glides: There is some reason to believe that within 

 the continental delta deformation may so accent the slope that 

 glides of unindurated sediments result. In explaining the Devo- 

 nian breccia of Iowa, McGee 3 offered as "a useful even though a 

 far-fetched hypothesis" that of an elevation at the close of the 

 Devonian by which the declivity was increased, a consequent slight 



1 The Califormia Earthquake of April 18, igo6, I, Part II, pp. 384 f. (Carnegie 

 Institute, 1908); Darwin, Voyage of a Naturalist (London, 1891), p. 220; Whymper, 

 Travels amongst the Great Andes, IV (London, 1892), 260; Whitman Cross, "Geology 

 of the Rico Mountains, Colorado," U.S. Geol. Surv., 21st Ann. Rept., Part II, p. 149. 



2 John Milne, "Suboceanic Changes," Geog. Jour., X, 129-46, 259-85. 



3 W. J. McGee, "Pleistocene History of Northeastern Iowa," U.S. Geol. Surv., 

 nth Ann. Rept., p. 323. 



