﻿THE CURRENTS OF THE OCEAN. 151 



Below the stratified drift there may be seen at two points, Sankaty Head (Nan- 

 tucket) and Truro (Cape Cod), a stratum of clay, which Mr. Desor and Mr. Edward 

 C. Cabot regard as the eastern outcrop of a wide tertiary basin, extending below the 

 islands of Nantucket and Martha's Vineyard, and forming the nuclei for the overlaying 

 drift-beds when they were deposited in the condition of shoals. 



There is some reason for assuming a similar superposition of strata of different ages 

 in the great banks to the northeast of the continent. It is certainly not a matter of 

 chance that there is no tertiary and so little stratified drift to be found north of Cape 

 Cod. In order to account for their absence, we must admit that there has been formerly 

 a far greater amount of dry land than now, which is not in accordance with the general 

 features of the country; or we must conclude that in those periods the tidal current was, 

 as it is now, too rapid to allow the formation of shoals along the coasts of Maine, &c, 

 and that the materials held in suspension were consequently carried still further, and 

 dropped in those great seats of deposit, George's, Newfoundland, and other banks in 

 that vicinity. This assumption helps to explain both the extent of these banks and 

 their height above the bottom of the ocean, which cannot without apparent difficulty be 

 ascribed merely to the tidal deposit of the actual period. But as the quaternary de- 

 posits, being the last in order, have been accumulated on the northeast, so the allu- 

 vium, carried beyond them by the new course given to the tide-wave, is seen in the 

 greatest quantities in the vast subaqueous deposits of the Nantucket shoals and banks, 

 the great and little banks of George and Newfoundland, and those off the coast of Nova 

 Scotia, before enumerated. 



Passing from the coast to the interior, this theory of tidal action serves to explain the 

 geological peculiarities of those great plains both of North and South America, the 

 prairies and the pampas, which may be considered the deposits of gigantic bays during 

 the state of subsidence of the continent. 



On the continent of Europe and the British Islands, the primary rocks of Portugal 

 and Spain, and the primary and secondary formations of the French and British coasts, 

 have a general similarity of outline to the present shores. Within these limits, therefore, 

 the transmission of the tide-waves has remained very nearly the same. Upon examin- 

 ing the geological map, a break is found to occur in these early formations, reaching from 

 the western vale of the Lower Pyrenees to the entrance of the Gironde. This, which is 

 the alluvial coast of the Bay of Biscay, noticed in the preceding section, is the external 

 boundary of a vast basin of tertiary and drift deposits, the former being inside. Mak- 

 ing a curve from Limoux towards Montpellier, the middle tertiary extends in a narrow 

 belt between the chalk on one side and the transition series on the other, until it meets 



