THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. V. 



No. v.— MAY, 1898. 



OI^XG-Zl^^JLIJ .A-I^TXCLES. 



I. — Professok J. W. Spencer on Changes of Level in Mexico. 

 By Prof. Edwaed Hull, M.A., LL.D., F.E.S., F.G.S. 



THROUGH the courtesy of Professor J. W. Spencer, I have 

 received an early copy of his paper on the " Great Changes of 

 Level in Mexico and the Interoceanic Connections," ^ containing the 

 observations and conclusions derived from a visit paid to this region 

 in 1895, with the special object of determining on the spot whether 

 the suggestion that the drainage of what is now the Gulf of Mexico 

 formerly crossed the Teliuantepec Isthmus into the Pacific. This 

 paper is not less interesting and important than the previous memoir 

 by the same author on the " Eeconstruction of the Antillean Con- 

 tinent," a resume of which has appeared in the Geological Magazine 

 (April, 1895) by the pen of Mr. A. J. Jukes-Browne ; but in this 

 latter case Professor Spencer was dealing mainly with facts and 

 inferences based on physical features under submerged areas of the 

 Atlantic Ocean, while in the present case he has to deal with 

 phenomena visible to the eye of the observer. 



It will be recollected that in expounding the hypothesis of 

 a former Antillean Continent the idea of a submergence of the 

 Mexican region and the Isthmus of Tehuantepec, sufficient to allow 

 of interoceanic communication, was in the author's mind a necessary 

 corollary. The "drowned rivers" and their present affluents, such 

 as the Mississippi, being shut out from the Atlantic, required an 

 outflow in the opposite direction into the Pacific. The position of 

 this outflow Professor Spencer has now determined with what 

 appears absolute certainty in the "divide" of the Tehuantepec 

 Isthmus at levels of about 1,000 feet above the ocean ; but before 

 describing this channel of interoceanic communication in more 

 detail, some account must be given of the Mexican topography, as 

 very clearly described and illustrated by photographs and drawings 

 by Professor Spencer. 



The region presents the aspect of a series of steps or plateaux, 

 from the most elevated at a height of 10,000 to 11,000 feet above 

 the ocean down to the coastal plains, forming, in the author's view, 

 "base-levels of erosion," and breaking off in escarpments which are 

 intersected by ravines or canons, cut back by the rivers to greater 

 or less distances into the plateaux themselves. The levels differ 

 ' Bull. Geol. Soc. Amer., vol. ix, pp. 13-34. 



DECADE IV. VOL. \. — HO. V. 13 



