280 NINTH ANNUAL REPORT OF 



Cruz ; and if the Sierra Madre has a northern equivalent, it must be 

 recognised in the Sierra Nevada, the Cascade Mountains, and their 

 more northern continuations : because the first line forms the eastern, 

 the second line the western borders of the great longitudinal basin of 

 our western interior, the whole construction being thus under the rule 

 of a strict physico-geographical analogy. 



8. Though in respect to its prevailing elevation, this great basin may 

 be called a plateau or table-land, still it has considerable differences of 

 altitude, and three great slopes — not to speak of similar phenomena of 

 a minor importance — which form transitions from the inner and higher to 

 the outer and lower countries: that of the Rio Grande, thatof the Colorado 

 and Gila, and that of the Columbia — the former breaking through the 

 western marginal chain. 



Between the middle part of the valley of the Rio Grande and the 

 middle part of the valley of the Gila, the country is less elevated than to 

 the north and south of that line. The level o^Lake Guzman, situated west- 

 southwest of El Paso, is, according to Mr. Schuchart, even lower than 

 that of the Rio Grande at El Paso^ Lake Santa Maria must have 

 about the same leveL Into this latter lake the Tlio Mimhres, which 

 comes from the north, is said to empty in time of copious rains ; while 

 from the south the Rio de Santa Maria, emptying into the same lake, 

 rushes down from the central plains of Chihuahua. A line traced from 

 these two lakes to the Dry Lagoon of Cook's route, forms a north- 

 western continuation of this depression of the table-land ; and from the 

 latter place the middle part of the Gila may be reached without over- 

 coming any considerable elevation, which, however, would be found 

 to exist to the north as well as to the south of that line. The upper 

 Gila runs in a narrow part of the higher country north of it ; and 

 though its bottom may be even lower than the level of the open country 

 along the general line of depression, still that does not form an objec- 

 tion against the general construction, as it has neither an opening to 

 the Rio Grande nor is it accessible much higher up than where the 

 road from Tucson first strikes it. If Cook's wagon route, in taking 

 from Dry Lagoon a southwestern course to the Guadalupe pass, 

 deviates to the south of our line, it is because it follows a series of fine 

 watering and pasture-places, situated just between the mountains of the 

 highest section of country, M'hich contains the origin of the southern 

 affluents of the Gila and of the northern river of Sonora. 



9. Thus it would appear that an ocean of a level not much higher 

 than the Rio Grande near El Paso would separate Mexico from the 

 rest of North America. 



But an ocean of that level — setting aside the more important changes 

 it would produce in the form of our continent — would cover the Colo- 

 rado desert, and, extending over the deep mountain passes southeast 

 of Los Angeles, would gain the Pacific here, and make an island of 

 Lower California. 



It is very possible that such a state of things has really once existed. 

 The nearly horizontal strata of the cretaceous formation of Texas 

 appear to enter in a western direction and unconformable superposition 

 between elevations of other sedimentary rocks and granite, syenitic, 

 porphyritic, and trachytic mountains, which must have already existed 

 when, and must have been above the surface of the ocean in which 



