THE SMITHSONIAN INSTITUTION. 281 



the cretaceous strata were deposited. Strata of that formation, in un- 

 conformable superposition, appear to exist at several places between 

 upheaved and eruptive tracts of country, in northern Chihuahua and 

 Sonora. And if a closer geological investigation should really prove 

 that, a little south of the upper and coinciding with the lower Gila, a 

 branch of the ocean should once have formed a strait across what is 

 now forming our present continent, we might say that some hundred 

 thousand years ago the natural line of a railroad, which in our days 

 should connect the eastern and western side of that continent, was 

 already traced by nature. 



It is an interesting fact that the desert north of the Lower Colorado, 

 which is in the western continuation of that old range of lower country, 

 is, even now, perhaps, the lowest spot of the American continent — as, 

 according to recent measurements, it is in part even somewhat under 

 the level of the ocean. While travelling through that country, I was 

 struck by certain phenomena connected with the periodical filhng and 

 drying of what has been called Neu River, and of the several lagoons 

 connected with it. The immense mud deposits of Little Lagoon, 

 which I have examined, prove the former existence of long and unin- 

 terrupted periods in which the water of the Colorado entered the 

 desert and kept the bed of New River, together with the basins of this 

 lagoon, full; while the existence of mezquit trees, now killed by its 

 water, from which the upper parts of their trunks and branches 

 emerge in a dead state, proves that otlier uninterrupted periods have 

 passed when the water of the Colorado did not enter the desert. Now, 

 it has been asserted that these fluctuations are the consequence of the 

 more copious or more scanty rains in the countries drained by the 

 Colorado and its tributaries; but the fluctuations appear to have 

 been of such an extent in time and level, that the cause assigned to 

 them appears to me to be inadequate to the effect, and I am more 

 inclined to believe that the phenomenon is, at least in part, produced 

 by fluctuations of the ground in consequence of the action of subterra- 

 nean forces. There is a large solfatara even now in action at the 

 northern side of the Lower Colorado. 



10. But to return to my strictly geographical object: It follows from 

 the foregoing statements and remarks that the great longitudinal basin 

 which constitutes the inner part of the western section of our continent, 

 is divided, by a depression of soil which runs from the Middle Rio 

 Grande to the Middle Gila, into a northern and a southern table-land, 

 the former being that of New Mexico, Utah, Upper Oregon, and other 

 more northern countries — the latter that of Mexico in its present 

 confines, as they have been fixed by the Gadsden purchase. At 

 the same time it can be seen how great an error it is, affecting 

 the whole physical geography of the continent, to bring the Sierra 

 Madre into connexion with the Rocky mountains. It makes the western, 

 marginal chain of the southern to be the continuation of the eastern 

 marginal chain of the northern half of the great longitudinal basin, 

 separating analogous and confounding heterogenous phenomena of 

 orography, of climatology, and of the distribution of vegetable and 

 animal life. Those who have studied the climate, and the flora and 

 fauna of these regions, will find that I am right in my assertions. 



