SOS C. Davison — On Mountain Evolution. 



The edges of the Mexican table-lands (a term more correct than 

 elevated valleys) show the counterparts of the submerged valleys 

 to , the east — an analogy not found in the valleys of the eastern 

 part of the continent — for the now unsubmerged portions of the 

 continent were too far from their margins to have been incised by; 

 the growing canons of the period of great elevation. Yet these 

 Mexican examples represent a shorter duration of time than the 

 drowned Antillean valleys. 



In Mexico youthful terraces occur to an elevation of 6,500 feet, 

 above the sea, and base planes of erosion upon the margins of the 

 Mesas to an elevation of 8,000 feet. With this enormous elevation 

 the terrace materials in the older valleys have not been removed by 

 denudation except in part, and the youthful canons have not yet 

 reached far into the plains. Thus it appears that Mexico has risen 

 to this great altitude in very recent times, and, when we correlate 

 the geological foundations, it would appear that Mexico and Central 

 America have risen to almost as great an elevation as the late 

 altitude of the Antilles and eastern part of America, while the floor 

 of the Mexican Gulf has been sinking. 



Across the floor of the divide between the Atlantic and the Pacific, 

 in the Isthmus of Tehuantepec, there were recent shallow straits, 

 succeeded by natural canals covered with level gravel floors con- 

 tinuous with the terraces on the Atlantic side. The importance lies 

 in the admittance to the Gulf of the sparsely-distributed Pacific 

 littoral types of molluscs. But the gravel floors of the channels 

 over the divides form a most important analogy. Such floors over 

 the watersheds of the Great Lakes of North America have been 

 regai-ded by some glacialists as evidence per se of glacial dams 

 confining the waters of the lake basins at high altitudes. Against 

 this view there are many considerations, but now that the same 

 phenomenon is found within a few degrees of the Equator, and at 

 low elevations, the value of this test for glacial dams disappears. 



While no phenomena observed in the Antillean region have 

 weakened the hypothesis of the great changes of land and sea in 

 recent times, yet there is much detailed evidence supporting the 

 theories set forth in the "Reconstruction of the Antillean Continent," 

 of which the points mentioned have a most important bearing, as 

 filling important gaps in the chain of evidence, concerning which 

 we had not the direct observations before the present time. 



VI. — Second Note on the Expansion Theory of Mountain 



Evolution. 



By Charles Davison, M.A., F.G.S., 



Mathematical Master at King Edward's High School, Birmingham. 



TN a note published four years ago,^ I pointed out a fundamental 

 objection to the principle of the expansion theory. That 

 objection has been clearly expressed as follows by Prof. Leconte,'^ 

 who, like myself, considers it fatal to the theory : " Sedimentation 



1 Geol. Mag., Vol. VIII, 1891, p. 210. 



2 Journ. of Geol., vol. i, 1893, pp. 570-571. 



