258 GEOLOGY. 



Gold range, or 50,000 feet to the Keweenawan, and when these systems 

 show signs of deposition in comparatively shallow water, it is commonly 

 inferred that the bottom of the basin of deposition sank during the 

 deposition to an amount corresponding approximately to these great 

 thicknesses. Now these are depths much greater than the deepest 

 parts of the oceans. Moreover, they are depths where high tempera- 

 tures should be experienced, 1 and where the pressure should exceed 

 the elastic limit of all known rock (estimated at about 30,000 feet), 2 

 and all crevices and pores should be obliterated and the rock particles 

 pressed into complete continuity, and highly metamorphosed. In the 

 face of these rather startling inferences, two attitudes have developed. 

 On the one hand, the correctness of the measurements or estimates of 

 these great thicknesses has been questioned, as the basal parts of the 

 thick series do not show these effects. On the other hand, accepting 

 the measurements, in the main, far-reaching theories relative to crustal 

 deformations and internal dynamics have been built upon the supposed 

 deep depression of the surface. It is therefore important to inquire 

 whether the fundamental assumption involved is correct, viz., that 

 the thickness of a series of beds deposited in a body of water is a true 

 measure of the depth of the basin receiving the deposits, or of the 

 amount of its depression while receiving them. 



One phase of the case may be clearly seen by consulting Fig. 108, 

 in which the usual conditions for the deposition of sediments on the 

 border of a continent are represented. When the sea covers the con- 

 tinental shelf, deposition takes place both on it and on the slope to the 

 abysmal depths of the ocean, and very slightly on the deep bottom of 

 the ocean. When the sea is withdrawn from the surface of the conti- 

 nent, the shore stands near the upper edge of the abysmal slope. That 

 this has recently been the case is shown by river-channels that extend 

 across the upper surface of the present continental shelf, and have their 

 termini on the upper border of the abysmal slope. The main deposition 

 is then on the slope, while slight deposition continued on the abysmal 

 bottom. As deposition on these abysmal slopes has gone on through- 

 out the whole history of the ocean basins, very extensive series of beds 

 have been laid down. Because of the different attitude of the sea to 



1 See Dana's Manual, pp. 451 and 469. 



2 Van Hise and Hoskins, 16th Ann. Rept. U. S. Geol. Surv., also this work, Vol. I 

 pp. 218, 219. 



