OROGRAPHIC ^rOVEMENT IX BONNEVILLE AREA oG5 



outlets. Lakes without outlets, and especially bodies of water standing 

 in dry regions, have both of these factors made a function of the rate of 

 evaporation, which is always higlu It is probable that in the case of 

 Lake Bonneville the prism of water present at any particular time is 

 capable of mathematical calculation. AVhen the inflow was at its maxi- 

 mum, tbe waters, owing to tlie peculiarities of the local relief, sprearl out 

 over tlie adjacent plains until an area was attained the evaporative co- 

 efficient of wliich exactly balanced at any particular moment that of the 

 intake. With so high an evaporative value as SO to 100 inches a year and 

 with so vast an area of sluillow waters, the fluctuations of the lake level 

 must have been relatively rapid and numerous. 



That Lake Bonneville was fed l)y streams other than those now trilui- 

 tary to Great Salt Lake, and that the headwaters of the pre-lacustrine 

 Virgen River drained into the basin and indeed formed and mainly sup- 

 ported the old lake, is evidenced by many facts. When, however, the 

 princij^al feeder was diverted, the lake waters soon fell in consequence 

 1,000 feet. 



To just what extent the diversion of the headwaters of the Old Virgen 

 River was due to diastrophic movements and what to volcanic disturb- 

 ances is now difficult to estimate. The last mentioned cause is probably 

 the determining one; but the first-named one by itself appears to be per- 

 fectly competent to accomplish tlie results. 



MAGNITUDE OF ^ECEf^.^ARY MOTEMEXTfi 



In the north the diversion of the Snake River headwaters back and 

 forth between its present channel and the basin of Great Salt Lake may 

 be readily accounted for by a differential movement of less than 250 feet. 



In the south the amount of movemeiit necessary effectually to barri- 

 cade the old Avatercourse is considerably greater. It was at least 1,200 

 feet, as shown by the elevation of the highest shore terraces. In reality 

 it was somewhat more than this figure. 



Altogether the necessary crustal oscillation is so surprisingly small as 

 to be almost beyond expectation. 



A^OLCAxic Disturbances affecting Lake Conditions 



CnOKTNG OF MAIN TlUmiTATilEH BY TjAYA FLOWH 



The volcanic phenomena in and a!)oiit lionneville basin are both 

 numerous and extensive. The principal expression of the more recent 

 activities of this kind are the basaltic streams. In point of time these 

 extend backward from almost the first settlement of the reo^ion to far 



