KHyjliMS IN DPJXIDATTON 



771 



the stage of old age, great areas oi' Paleozoic and Mesozoic sediments will 

 iiave become destroyed. But nature reveals while she destroys, and be- 

 ca;use of this very fact of the contrast of the [present to the past the com- 

 prehensive study oT the stratigraphic record is made \ery much easier 

 than it would have l)ecii in the a\'erage coiidition of the lands, and in this 

 particular century has been further facilitated through the fresh ex- 

 posures made hy man incidental to his development of transportation 

 systems. 



The existence of a mantle of sedimentary rocks, mostly marine, cover- 

 ing three-(.|uai'tej-s of the land surfaces, proNcs that since tiie opening of 



-3,000. -• 



Figure 3.- — Hypsoyiaphiv Cure e of the Continents and its Relation to the Paot 



A. Curve of continental relief, showing relations of altitude to area. 

 R. Curve giving one-fourth relief, representative of periods of wide epeiric seas. 

 '■- Limicii of periodic oscillations of sealevel to flood two-thirds of continental area with 

 profile B. 



the MidcUe Cambrian these portions. of the . continents have, on the whole 

 receiyfid sedinn^nt rnther than, supplied it. Of course, there has been 

 alternating deposition and. erosion,, I Jiiknown amounts are gone, but the 

 balance was in favor of accumulation of sediments. . Marine shales, and 

 limestones are the most \A"ide-spread of formations. Through most of 

 geologic time .the continents have lain awash with the ocean level and 

 across them the rising ocean floods have flowed. The sediment' of these 

 Paleozoic anrl Mesozoic times came in o-reatest bulk from the limifp'd 



