HOW FAR VEGETATION IS A PROOF OF RAIN FALL. 173 



drawn from sucli results, and applied to the interior of a continent, under latitude 32°, must be 

 erroneous, unless the considerations to which I have alluded be attended to.* 



Nothing is more common than the loose verbiage in the mouths of many, that, the geological 

 circumstances being favorable, artesian wells are possible to be produced. 



This may be true in north Europe and along our Atlantic coast, where the rains are constant, 

 and to a high figure ; but it is not true in the Gadsden Purchase, because the climatal relations 

 approach that of the tropical zone. 



An objection may be raised to the arguments advanced and the conclusion drawn, based upon 

 imperfect observations of the climate. It may be said that the registers of Fort Yuma, and 

 those on the Rio Grande, afford no index of rain fall of the country between these points, and 

 that, as the country inland is more elevated, it is but reasonable to suppose that a large supply 

 of rain does exist. The reply to this objection is, that no matter how ignorant we may be of 

 the actual rain fall, as estimated by a rain gauge, yet that there are always evidences suffi- 

 cient in the district furnished at once, and visible at a single inspection, which can lead to an 

 approximation of the actual fact. These evidences are, the appearance of the water-courses, and 

 the character — the species — of the vegetation natural to the district. 



If the water-courses be short, if the bed be rugged, filled with large stones, washed out from 

 above and impacted in gravel, and if it suddenly terminate by opening upon plain land, and 

 does not empty into another channel or river bed, then it is the course of a short and tumul- 

 tuous body of water, collected together suddenly, and rushing along in a mass, and not in a 

 continual stream — it is a torrent and not a river. It does not signify how deep the sides of the 

 water-courses are — these do not indicate long continued and low action ; a small creek, sud- 

 denly emptying into the Hudson river below Newburg, has (in 1852) in one night excavated 

 a bank 60 feet deep ; it is but the evidence of force or power of water, but not of its actual 

 quantity, or of its continuance of action. 



Especially is this true where it does not empty into a larger stream bed ; it shows that the 

 total quantity has been too small to prolong it into a stream. Numberless are the arroyos or 

 creek beds of this character, which give out, or lead to nowhere. 



Again, with regard to the evidence derived from vegetation. This is more certain than the 

 other. Let us consider a district which has no subterranean supply of water, and wholly 

 dependent upon the rains. Suppose such a district to be wholly destitute of vegetation, the 

 observer would infer either of two things : 1st, that no rain at all fell, or that it did not fall at 

 the time favorable to vegetation. The first supposition is improbable, as we know there are 

 few places on the globe where some rain (slight though it be) does not fall ; and it is not true 

 of any locality on the route, for in some of the most barren localities the arroyo beds were 

 found ; and in others, as the playas, the surface was ripple-marked by superjacent water. 

 With regard to the second supposition, that it did not fall at the favorable time to benefit vege- 

 tation, it may be worth while to inquire into its meaning : Is there any time peculiarly favor- 

 able to vegetation ? are plants so obstinate, and such formalists, as to grow in exact months in 

 spring time when it does not rain, and refuse to grow in summer when it does? 



Our knowledge of vegetation shows that plants will grow when the supply of moisture in the 

 ground is sufficient to carry the food it requires in a liquid state to its roots ; it cannot grow 

 until after some rains, nor mature itself until after more ; and that, let it be supplied with 



" The artesian borings, as tiiey are called in California, are not cases in point ; they are, in fact, not artesian wells, but 

 simply deep clay borings. 



