Piet 
* 
J. W. Powell's Survey of the Rocky Mountain Region. 351 
sage brush. The whole period is as likely to have been meas- 
ured by centuries as by decades. 
Thus it appears that the last twelve years have witnessed an 
extension of the lake, which is not only without precedent in 
the experience of the citizens of Utah but is clearly an anomaly 
in the history of the lake. To explain it and to explain at the 
same time the increase of the streams, there are two general 
theories worthy of consideration. 
he first is that there has been a change of climate in Utah 
whereby the atmosphere is moister, so that the fall of rain and 
snow has become greater and the rate of evaporation has be- 
come slower. The second is that the industries of the white 
man, which have been steadily growing in importance for the 
last thirty years, have so modified the surface of the land that 
a larger share of the snow and rain finds its way into the 
water-courses and a smaller share is returned to the air by 
thus check the evaporation from their surfaces, and the streams 
which he thereby rescues from dissipation are used In irriga- 
tion for a few months only, while for the remainder of the year 
they pay their tribute to the lake. The destruction of grasses 
he 
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increase. : 
- Geological work by Mr. Gilbert—During the preceding sum- 
mer Mr. Gilbert had discovered a peculiar series of phenomena 
