i894. CONTINENTAL GROWTH. 34^ 



Mexico, is Tertiary (Eocene and Miocene), and a smaller portion 

 consists of Quaternary deposits. This would seem to show that for a 

 considerable period of time after the upheaval of the Rocky 

 Mountain region continental conditions prevailed over this area, and 

 that the sediments from its erosion and waste now lie at the bottom 

 of the Atlantic. 



Unfortunately we cannot read the earth's history with any fulness 

 or accuracy. We may only see as through a glass darkly, but still 

 we are justified in concluding from a consideration of these examples, 

 which could be multiplied from well-nigh all the known world, that an 

 enormous erosion of the land has taken place during the Quaternary 

 period, and that these sediments, probably with those of an earlier 

 period, now lie on the ocean floor, are still accumulating, and will 

 accumulate until such time as the earth's living forces bring them up 

 from below the waters to take their place as mountain, plain and 

 valley in the unceasing cycle of the earth's changes. 



Conditions of the Earth's Crust in which Sediments 

 Accumulate, and Rate of Accumulation. 



The conditions of the crust of the earth upon which these sedi- 

 ments are being laid down are as varied as the sediments themselves. 

 While in the north-eastern portions of North America, from New 

 York to Baffins Bay, there are no evidences of volcanic activity either 

 in the present time or late geological past, south of New York we 

 gradually approach a volcanic and earthquake area, crossing the Gulf 

 of Mexico, and culminating in Central America. The West India 

 Islands give frequent evidence of volcanic instability by the raised 

 coral formations which are there met with, together with foramini- 

 feral deposits considered to be of deep-sea origin. ^^ Within 

 this basin-shaped and almost closed Gulf of Mexico, deep-sea 

 oozes are being laid down, and into this area, at one locality or 

 another, the Mississippi has delivered its daily burden of sediment 

 through, at least, Quaternary time. These spoils of the continental 

 land will probably be interbedded with the lime deposits worn from 

 coral reefs and with the more purely oceanic accumulations of 

 foraminifera and other deep-sea forms of life. ^9 



Fluctuations of level of the Mississippi mouth and valley may 

 have given the terrigenous deposits a wider distribution than what 

 obtains now. Borings in the Mississippi Valley show a subsidence of 

 at least 630 feet at New Orleans. 



If the land were upraised so as to represent the physiography of 

 the time of this elevation, the sediments now being brought down 

 this great drainage channel would be delivered further into the 



18 •' On the Elevated Coral Reefs of Cuba," W. O. Crosby, Pvoc. Boston Sac. oj 

 Nat. Hist., vol. xxii. ; " The Geology of Barbados," Jukes-Brown and Harrison, 

 Q.J.G.S., 1892, pp. 170-226. 



1'-' S^e three letters by Ale.x. Agassiz — "On the Dredging of the U.S. Steamer 

 ' Blake,' " Bull. Museum of Comparative Zoology, Harvard, vol. v., 1878., 



