HEAVY RAIN-FALLS IN DRY REGIONS. 179 



those of the tropics, Avhere it is with the greatest difficulty that forests 

 Clin be kept from encroaching on the clearings ; or we may have those, like 

 the Great Basin, where arboreal vegetation hardly exists at all, and where 

 what little does exist is with difficulty replaced when it has once been re- 

 moved. Countries in the latter category are, of course, those where the 

 precipitation is already very small and still diminishing. In such cases we 

 may easily conceive that the existing scanty forest represents a past condi- 

 tion of things rather than the present one. 



That where the conditions are as thus indicated the question of preserving 

 the still remaining forest growth may become one of some importance can- 

 not be denied. Just as a man in the desert with only a scanty supply of 

 water would husband it by putting it in the shade, rather than allow it to be 

 exposed to evapoi-ation in the direct sunlight, so we may suppose that the 

 total disappearance of springs may be retarded in a desiccating country by 

 carefully preserving the forests which still cling to the shady nooks and 

 recesses of the mountains. That the total amount of precipitation might thus 

 be increased in any perceptible degree is not at all probable ; it is simply a 

 method of economizing that which does fall, and as such might be of impor- 

 tance. It is also generally conceded that in ver^^ dry countries the small 

 rain-foil which does occur is liable to come in the most irregular manner, 

 and there is a strong disposition to ascribe this to the absence of forests. 

 Inasmuch as very dry countries are usually treeless, the association of irregu- 

 larity of rain-fall with absence of forests is a very natural one ; but, as has 

 already been seen, treeless regions may be abundantly watered, and then 

 precipitation has its usual regular course. The astonishing amount of water 

 which can be precipitated, within a short space of time, in a region where 

 the total average rain-fall is very small, and where the country generally has 

 the aspect of entire dryness, is something wonderful to behold. Such violent 

 falls of rain are popularly known in the Cordilleras as " cloud-bursts." Of 

 course any one such phenomenon is limited to small areas ; but if it takes 

 place wdiere the topographical conditions are favorable, so that the waters 

 are confined in their course, persons may be and have been — to the writer's 

 knowledge, in more than one case — drowned on a spot where but a few 

 minutes before all w'as absolute dryness. The nature and cause of these 

 cloud-bursts seems to be but imperfectly understood ; but the constant 

 association of intense electrical displays with them would seem to indicate 

 a causal connection between the two sets of phenomena. 



