ITS MbGee — Gulf of Mexico as a measure of Isostasy. 



The earlier and most of the later data upon which the doc- 

 trine of isostasy depends were indirect, i. e., they were infer- 

 ences from the characters and relations of formations laid 

 down, or terranes lifted and degraded, during long past eons ; 

 yet there is no dearth of direct data — i. e., data derived imme- 

 diately from observation — sustaining the doctrine. 



One class of such data is found in the relation between 

 tracts of deposition and earthquakes : If the earthquakes re- 

 corded in history are separated into three categories of associa- 

 tion, viz : (1) earthquakes associated with volcanoes, (2) earth- 

 quakes associated with hot springs, and lines or zones of active 

 orogeny, and (3) great earthquakes apparently not connected 

 with vulcanism or orogeny- — it will be found that the greater 

 part of the third category have affected tracts of rapid deposi- 

 tion, and commonly that after the tremor the land stood lower 

 than before. Thus, the Charleston earthquake of 1886, felt 

 over an area of nearly a million square miles, affected the 

 Atlantic coastal }3lain of the United States, a tract of remark- 

 able simplicity and uniformity of movement during later 

 geologic times yet one of rapid and long continued deposition ; 

 the New Madrid earthquake of 1811-13, felt over an area of 

 certainly a million and a half square miles in central and 

 eastern United States, affected a tract of exceptionally gentle 

 movement and uniform geologic history, though the unloading 

 ground of the great river of the continent, and the land was 

 spasmodically depressed o,ver thousands of square miles ; the 

 great Lisbon earthquake of 1755 originated in and affected 

 most disastrously the tract upon which the Tagus drops the 

 detritus gathered from a sixth part of the Iberian peninsula ; 

 the Ivach earthquakes of 1819 and later dates devastated the 

 delta of the mountain-born Indus and left the land some feet 

 or yards lower than before, thus extending the vast watery 

 waste known as the Rann of Kach ; the Cachar earthquake of 

 1869 similarly affected the deposition-tract of the powerful 

 Ganges and Brahma-putra ; the deposition-tract of the mud- 

 stained Hwang-ho and the torrential Yang-tse-Kiang are notori- 

 ously earthquake ridden — in short, nearly all if not all of the 

 extensive non-volcanic earthquakes recorded in history cen- 

 tered in tracts of rapid deposition. In some cases the data 

 derived from this relation are equivocal ; yet they are some- 

 times apparently trustworthy. 



A second class of direct data sustaining the doctrine of 

 isostasy is found in the relation between tracts of deposition 

 and measured subsidence of the land or encroachment of the 

 waters. If the tracts of rapid deposition are separated into 

 three categories of association, viz : (1) deposition tracts within 

 or near the areas of Pleistocene glaciation, (2) deposition tracts 



