Semi-arid Portion of Southwestern Neto Mexico. 245 



of a region for a period of 40 years, more or less, has produced 

 so great results, it requires no great stretch of the imagination 

 to picture the conditions in this section a few centuries hence 

 if the same processes continue, as there is every reason to 

 believe they will. 



It is not alone within the area of the Silver City quadrangle 

 that stream trenching is a conspicuous feature, for it is com- 

 mon in many parts of the west. In traveling through the 

 states of New Mexico, Colorado and Wyoming, one finds that 

 this trenched condition is widespread. The recency of the 

 trenching is indicated by the fact that it is still in progress, and 

 the gulleys are still working back towards the heads of the 

 smaller tributaries. 



In making a wider application of the principles of stream 

 trenching which we have outlined, it is evident that, since the 

 trenching is the result of the removal of vegetation cover, we* 

 will be likely to find it only in those regions whose normal 

 climate is such that the normal vegetation would be luxuriant 

 enough to become an important factor in the conservation of 

 the rainfall, and in the protection of the valley bottoms from 

 direct erosion. In such regions the overstocking, to which the 

 west has been subjected, will have so reduced the cover as to 

 cause increased floods with accompanying stream trenching. 

 In regions normally too arid for the formation of an efficient 

 vegetation cover the trenching should not be apparent. 



Within the area of the Silver City quadrangle alone there is 

 some evidence bearing on this point. The extreme southeast- 

 ern corner of the sheet is lower and much more arid than the 

 rest of the area, and it was noted that here the trenching is not 

 so well developed as in the portions of the quadrangle which 

 receive a greater precipitation. In order to thoroughly test 

 the principle, comparison should be made between portions of 

 the state, or of other states, which have climatic conditions 

 similar to those of the Silver City region and other portions 

 where climate is distinctly more arid, as, for instance, portions 

 of southern New Mexico and Arizona. 



