378 DYNAHnCAL GEOLOGY. 



the mass, while it might be comparatively rapid if a liquid layer existed 

 beneath a thin crust — a Jlotation crust, as it has been called. Darwin 

 has remarked that through molecular movements the earth's spheroidal form 

 might change with change of rotation. But what is the minimum limit in a 

 solid globe, to rate of adjustment — that is, to the rate at which resistances 

 from cohesion and other causes can be overcome — no known facts have even 

 approximately indicated. Effects should, in any case, lag behind the cause 

 of change, whether they are those from the deposition or removal of a load. 



There are, however, facts that seem to imply a somewhat easy adjust- 

 ment. Many low coasts over which sediments are borne to the sea border 

 are known to be slowly sinking ; as, for example, the coast of New Jersey, 

 where the rate, according to G. H. Cook, is two feet a century. This sink- 

 ing, and that of other parts of the Atlantic border, is attributed by Cook to 

 gravitation in the sediments. W. J. McGee, in a paper of 1892, has brought 

 together many facts from various coasts, mostly adjoining the mouths of 

 rivers, bearing in the same direction. On the Netherland coast, the rate of 

 sinking, according to Girard, is 0-09 to 0-75 meter per century, and 0-26 

 meter since 1732. But actual sinking is" not a legitimate isostatic effect. 

 The subsidence on such coasts corresponding to the amount of contributed 

 sediments (not exceeding it) is not indicated by the amount of sinking, for 

 the sinking is in excess of it. Other facts are more decisive. A boring on 

 the southeast coast at Atlantic City, 1398 feet deep, extended through beds, 

 as stated by J. C. Smock, which were proved by the fossils to be Miocene ; 

 Turritella plebia occurring, according to Heilprin, at 450 feet, and Perna 

 maxillata at 760 feet, of which depth 265 feet are surface gravels and 265 

 beyond are of doubtful reference. But at Asbury Park and Ocean Grove, 

 farther north, wells afforded the Upper Greensand with Terehratula Harlani 

 and other Upper Cretaceous fossils at a depth severally of only 270 and 280 

 feet below tide level, and the Lower Greensand at 365 and 382 feet. The 

 facts indicate a very slow rate of subsidence at Asbury Park since the Cre- 

 taceous period, and much less slow at Atlantic City, which is 80 miles south 

 of Asbury Park and only 40 from the north cape of Delaware Bay. A boring 

 on the coast of Texas passed through 3070 feet of shore deposits, without 

 reaching, according to the investigations of G. D. Harris, beyond the Miocene. 

 The deposits down to a depth of 458 feet are pronounced Quaternary. 

 Beyond, to the 1511-foot level no Tertiary fossils were found and all of them 

 may still be Quaternary. Between 1511 and 2153 feet, the deposits were 

 Upper Tertiary as shown by fossils ; and between 2153 and 2920 feet. Upper 

 Miocene. In the lower 150 feet, clays and sands were found without fossils. 

 Similar facts are reported from the delta of the Ganges and other regions. 



These proofs of rather rapid subsidence along coasts are regarded by 

 many as not inconsistent with the idea of a solid earth. Others have used 

 them as strong evidence of a thin flotation crust over liquid rock. 



But a "flotation crust" has its difficulties. The fact that there are high 

 mountains anywhere is one of them. Against this objection it is urged that 



