424 A TREATISE ON METAMORPHISM. 



(a) As a result of climatic changes there may be very great variations 

 in precipitation in different epochs in the same region. For instance, there 

 is every reason to believe that in the Great Basin during Glacial time pre- 

 cipitation was abundant, whereas at the present time precipitation is sparse. 

 At the epoch of abundant precipitation there were in this region vast 

 fresh-water lakes, such as Bonneville and Lahontan. At this time of 

 humidity it is highly probable that the level of ground water was compara- 

 tively near the surface, while at the present time for much of the region it 

 is a considerable distance below the surface. Similar climatic changes 

 affected much of the western part of the United States and have affected 

 other parts of the world at various periods of geologic time. 



(b) Besides these great climatic changes there are in many parts of the 

 the world alternating cycles of several years or a decade, during a part of 

 which there is more than an average precipitation, followed by other years in 

 which the precipitation is less than normal. At times of abundant precipi- 

 tation the level of ground water rises; at times of deficient precipitation it 

 falls. The rise and fall in the level of ground water due to such cycles 

 may amount to several meters. It not infrequently happens that wells sunk 

 during the humid part of a cycle have to be sunk deeper during the part of 

 the cycle in which the precipitation is small. During the closing half dozen 

 years of the nineteenth century California experienced the dry part of one 

 of these cycles. As a consequence of the deficient precipitation vegetation 

 very greatly suffered. During these years in southern California the level of 

 ground water markedly fell at various places from 5 to 10 or even 20 meters. 

 For instance, in the Los Angeles Basin, in the area above where the river 

 emerges from underground, during the years 1895-1899 the level of ground 

 water fell at the rate of between 0.3 and 0.4 meters per kilometer per annum 

 in passing upward from the source of the river. But in southern California 

 the fall in the level of ground water is very largely, due to the influence of 

 man. (See pp. 427-428.) Pumping of ground water to the surface for 

 irrigation, and not variation in precipitation, is one of the main causes of 

 this fall. 



(c) The greater part of the earth has either one or two seasons of 

 relatively abundant precipitation, followed by one or two less wet or dry 

 seasons; therefore the cycle of seasonal variation is semiannual or annual. 



