296 G. K. Gilbert — Inculcation of Scientific Method. 



bottom of Lake Bonneville was raised in connection with the 

 dessication of the lake, we have a true cause of upward move- 

 ment, and if we can show furthermore that the temperature of 

 the surrounding region was not equally raised, we have at least 

 a qualitative explanation of the differential uplift, the phenom- 

 enon to be accounted for. 



It is now several years since these explanations were first 

 suggested, and subsequent reflection has developed no others. 

 While all of them appear perfectly rational, only a very 

 slight inspection was necessary to raise a doubt as to the quan- 

 titative sufficiency of the second and third. It was therefore 

 determined to ascertain as accurately as possible the maximum 

 change which might be ascribed to each of the three suggested 

 causes, and to compare it with the actual change. The actual 

 change is susceptible of various statements. If we consider 

 only the measurements on the margin of the main body of 

 water, and in its center we find a difference of 100 feet ; by in- 

 cluding observations on outlying ba} r s we get a maximum dif- 

 ference of 168 feet ; and a study of the peripheral slopes of the 

 uplift suggests that they extend somewhat beyond the boun- 

 daries of the lake. Crude extrapolation gives 200 feet as a 

 maximum estimate of the height of the crustal dome. 



Take first the hypothesis that the crust of the earth, floating 

 on a molten nucleus, rose up in the region of the basin when 

 its weight was locally diminished by the removal of the water 

 of the lake. The weight of the load removed is measured by 

 the depth of the water before evaporation, 1000 feet. The 

 theory supposes that as the crust rose there flowed in beneath 

 enough molten rock to replace the weight of the evaporated 

 water. If the rock was very heavy, a layer of moderate depth 

 was necessary; if it was less heavy, more was required; but 

 in any event the thickness of the introduced layer must be 

 equal to the amount of the superficial uplift. It is known that 

 the density of the earth's material increases downward, for the 

 mean density of the earth, expressed in terms of the density of 

 water, is about 5*5, while that of the upper portion of the crust 

 is about 2 - 7. Nothing is known however of the law under 

 which the density increases, and nothing is known as to the 

 depth of the zone at which matter is sufficiently mobile to be 

 moved beneath the Bonneville basin. We may, however, in- 

 dicate limits, and I think this is fairly done by assuming that 

 the density of the introduced matter was not less than 3, nor 

 more than 5'5. If it was 5*5, the uplift consequent on the 

 evaporation of 1000 feet of water would be 182 feet. If it 

 was 3, the uplift would be 333 feet. Now it has already been 

 stated that the greatest value observation suggests for the 

 amount of the uplift is 200 feet. The postulate is therefore 

 abundant!}' competent in a quantitative way. 



