Revietcs — Dr. Sjoencer — The Antillean Continent. 173 



suppose, as he suggests, that the earth consists of a shell of rock 

 similar to what we have now at the surface, and of an interior mass 

 which conducts heat much more rapidly than the outer crust. 



Now, if this be the case, the eifectJ of the greater interior con- 

 ductivity will obviously be to keep the joining surface of the shell 

 and nucleus at a higher temperature than the surface at the same 

 depth in Lord Kelvin's imaginary earth. Near the surface of the 

 earth, thereibre, the rate at which the temperature increases with 

 the depth would be greater now in Prof. Perry's than in Lord 

 Kelvin's earth. But this rate decreases as the time since con- 

 solidation increases. Hence, a given downward rate of increase of 

 temperature (say 1° F. in 50 feet) would be reached much earlier 

 with Lord Kelvin's than with Prof. Perry's conditions. Li other 

 words, if Prof. Perry's supposition be correct, and on this point 

 most geologists will probably agree with him, Lord Kelvin's 

 estimate of the age of the earth errs by being far too small. 



How much too small cannot be determined, but it is sufficient for 

 geological purposes to know that the upper limit of 400 million 

 years may have to be greatly increased, and that one of the orders 

 to stop payment on the Bank of Time is for the present practically 

 withdrawn. C. IJavison. 



III. — Reconstruction of the Antillean Continent. By J. W. 

 Spencer, Ph.D., F.G.S. Bulletin of the Geological Society of 

 America, yoI. vi. pp. 103 to 140, January, 1895. With a 

 plate. 



DR. SPENCER was led to undertake this investigation by his 

 study of submerged valleys in higher latitudes, and he found 

 tliat similar evidence existed in the Antillean region. Recent 

 geological work in Florida and in some of the West Indian islands 

 led him also to visit Cuba and to study its geological structure. 

 One result of this visit has already been published in this Magazine, 

 and now we have another important contribution to West Indian 

 geology. 



The first part of the essay is a study of submarine geography : 

 the author points out that the continental shelf of the east coast 

 is continued in the broad plateau on which the Bahama Islands 

 stand, and that outside this upper shelf there is a much lower one, 

 termed the Blake Plateau by Prof. Agassiz, which lies at an 

 average depth of 2700 feet below sea-level; from this there is 

 a steep slope down to 12,000 feet (2000 fathoms). 



The outer margin of this lower shelf is indented by embay ments 

 from the deep water beyond, and the submarine plateau is trenched 

 by deep and narrow depressions which appear to be submerged 

 A'alleys' formed by river erosion, and these valleys open into the 

 embay ments. 



1 Dr. Spencer's application of the term fjord to these valleys is not quite in 

 accordance with European usage, for their coastal portions are filled with alluvium 

 and are not open inlets. 



