738 NATURAL SCIENCE. i^^^.. 



scarcely satisfactory, and sand-dunes do not necessarily indicate sub- 

 sidence. So much of Mr. McGee's paper is taken up with the attempt 

 to strengthen his case by the appeal to evidence of doubtful value^ 

 that we cannot ignore it ; the Gulf of Mexico, however, is the region 

 with which he is primarily concerned. To that area we will 

 now turn. 



The Gulf of Mexico is a nearly land-locked sea, which receives 

 the detritus brought by the Mississippi and other rivers and derived 

 from the erosion of about half the North American continent. This 

 detritus, however, is nearly all deposited in the northern half of the 

 Gulf, so the area of degradation is about 6i times the area of deposition.. 

 If the theory be correct, that the rise or fall of an area depends on 

 the weight removed or piled upon it, then the continent of North 

 America must be a rising area, and the northern half of the Gulf of 

 Mexico a subsiding one. 



Such is probably the case, but there is some danger of confusion 

 between cause and effect. It is quite as logical to argue that the 

 land is denuded because it rises, as that it rises because it is denuded^ 

 We may also observe that if from any cause the land sinks beneath 

 the sea-level, the part that subsides most rapidly necessarily becomes 

 first a bay and then probably an estuary, for river-valleys will tend 

 also to follow lines of depression. Suppose, on the other hand, that 

 the floor of the Gulf of Mexico were to rise ; the Mississippi would 

 discharge further to the east, beyond the West Indies, and we should 

 be told that the depression beyond was caused by the accumula- 

 tion of sediment. Unless the theory of stable equilibrium be of 

 merely local application, it is difficult to understand how land once 

 beneath the sea-level can ever rise again ; yet we know that extensive 

 marine deposits often occur high above the present sea-level. 



Mr. Guppy, writing of a district similar in many respects to that 

 dealt with by Mr. McGee, treats the subject from a point of view 

 altogether different. He is concerned mainly with the geological and 

 biological evidence, which go to indicate that what is now dry land 

 was once a deep sea, and the facts relied on are these : — Trinidad 

 possesses in its southern half certain Tertiary formations, which from 

 the character of their fauna seem to indicate a depth of at least i,ooo 

 fathoms. These deep-water deposits are absent from surrounding 

 areas ; and from their partial occurrence, combined with the evidence 

 obtained from the geology of the surrounding islands and coast of 

 South America, Mr. Guppy arrives at these conclusions. During the 

 Cretaceous and Eocene periods there was a sea having a depth of 

 i,ooo fathoms or more in the area now occupied by the microzoic 

 rocks of Trinidad. During the same period the northern mountains 

 formed probably an unbroken range with the littoral Cordillera of 

 Venezuela, and constituted the southern boundary of a Caribbean 

 Continent. These mountains may be regarded as one of those 

 " stable areas " which have never been submerged since Palaeozoic 



