MUSEUM OF COMPARATIVE ZOOLOGY. 141 



subject to energetic erosion have contributed their detritus to this basin. 

 The line of the xVntilles appeal's to me to mark the phenomena of coun- 

 ter-thrust due to the accumulation of deposits off the coast of South 

 America, much as the peninsulas of Florida and Yucatan mark the 

 effects of sedimentation in that sea and in the Mexican Gulf. To the 

 down-thrust caused by sediments derived from the island of Cuba, and 

 deposited on the sea-floor to the northward, we may perhaps attribute 

 the sudden termination of the ITorida elevation on the south. The 

 general tendency to ccunter-thrust uplift produced by the growth of 

 strata in the Gulf of Mexico, and manifested in the peninsula of Flor- 

 ida, is here interrupted by the process of local sedimentation. It is 

 probable that, at the present time, the considerable energy with which 

 the Gulf Stream moves through the Strait of Florida may hinder the 

 process of deposition of sediment derived from the Cuban land mass ; 

 but, as I shall endeavor to show in the sequel of this paper, this limita- 

 tion of the Gulf Stream is probably a matter of very recent geologic 

 time. 



Turning now again to the Cincinnati axis, let us note its relations to 

 the geography of the district at the time when it was formed, to see what 

 light it may throw upon the development of the curious elevations about 

 the Gulf of Mexico and the Caribbean Sea. The Cincinnati axis, as is 

 well known, is a singularly broad fold, which was developed on the floor 

 of the palseozoic sea at a distance of two hundred miles or more from the 

 then shore of the Appalachian Islands, and generally parallel with the 

 ancient land. At first, in the period of the Lower Trenton, this floor 

 appears to have been tolerably level. Before and during this period in 

 the history of the earth, a vast amount of detritus was borne from the 

 Appalachian land, and deposited on the sea-floor near its eastern shores. 

 Thus, along these old shores we had a vast thickness of sandstones of 

 the Okoee and Chilowee age, and above them a great thickness of rocks 

 belonging to the Knox group, which, though partly of organic origin, are 

 largely composed of inorganic waste from the old lands on the east. 

 There is no doubt that these last sediments were derived from the 

 Appalachian land, and they form an extremely massive system of sedi- 

 ments along the ancient shore. Following their onlaying, this portion 

 of the sea-floor which they occupied sank tf) a great depth, as is shown 

 by the peculiar character of the sediments and the organic forms in 

 the Trenton rocks of Eastern Tennessee. Apparently at this time the 

 Cincinnati anticlinal rose to near the surface of the waters, to a point 

 where it exposed the bottom of the sea to the action of currents siiffi- 



