Correspondence — Mr, A. J. Jukes-Browne. 143 



ELEVATION AND SUBSIDENCE IN CENTRAL AMERICA. 



Sir, — Allow me to point out that Mr. W. Upliam's letter in the 

 February Number of this Magazine is not an answer to my article 

 in that for December, 1890; because he completely ignores the point 

 of my communication and re-aseerts views which I did not question. 



In his original article of November, 1890, Mr. Upham cited the 

 Gulf of Mexico as having partaken in the high continental elevation 

 and in the subsequent subsidence to an extent of 3000 feet. In 

 December I quoted facts to prove that on the south side of the Gulf 

 of Mexico there has been a recent upward movement of probably 

 2000 ft. Hence if the Mississippi subsidence was contemporaneous 

 with the Cuban elevation, there must have been a differential move- 

 ment of 5000 feet within a comparatively small area. 



I do not say the difficulty is unsurmountable ; the axis of the one 

 oscillation may not have been located along the same parallel as 

 the axis of the other oscillation, or if Mr. Upham likes he can 

 postulate a great east and west fault through the Gulf of Mexico, 

 with a downtlirow on the north of about 5000 feet. The point, 

 however, does require notice, and it is not noticed at all in the 

 letter which purports to be a reply. I will therefore put the difficulty 

 more fully. During Pliocene and Pleistocene time there was in 

 North America a great elevation followed by a great subsidence ; 

 and in Central America a subsidence followed by great elevation. 

 If these movements were correlative, should we not expect to find 

 a zone where there was no movement at all ? Would not the 

 greatest vertical displacement be found in Canada or the Northern 

 States, and the least in the Southern States, and would not the 

 evidences of subsidence die away southward, and then be re^alaced 

 by evidences of upheaval ? Instead of this, we find the movements 

 almost at a maximum in those parts of the two areas which are 

 nearest to one another. Can Mr. Upham refer to any evidence about 

 alteration of levels along the west coast of the Gulf of Mexico ? 



In a paper just read before the Geological Society, I have adduced 

 evidence of the recent uprise of the Caribbean and Panaraic region, 

 and have advocated the view that the Gulf Stream passed into the 

 Pacific while the Glacial Period prevailed in the North Atlantic. 

 I am therefore strongly disposed to agree with the general views 

 put forward by Mr. Upham as to the geographical conditions that 

 prevailed during that period ; but in generalizing about the move- 

 ments which have affected such vast areas, we cannot be too careful 

 about the accuracy of the data on which the inferences are based, 

 and nothing is gained by ignoring difficulties. 



It was because I thought that this difficulty might not have 

 occurred to Mr. Upham that I put it before him; but I shall be 

 very pleased to see it answered in a manner that will strengthen 

 Mr. Upham's theory of correlative movements, and I hope Dr. 

 Spencer, who wrote much more cautiously than Mr. Upham, may 

 shortly have something to say about the date of the great depression. 



Teignmouth, February 1th. A. J. Jukes-Browne. 



