130 GROVE KARL GILBERT— DAVIS [MaMOIM [vo™xxt 



With the conclusions of his experiments in mind, Gilbert made a number of hypothetical 

 assumptions of greater or less plausibility in explanation of the supposed relation of lake con- 

 tinence to the deposition of the heavy clays, and of lake overflow to the deposition of the thin 

 marls. The assumptions may be presented in two groups. Those of the first group concerned 

 the composition of the lake in its earlier and later epochs of expansion: The lake waters in the 

 earlier humid period having no outlet must have been saline; during the intermediate epoch 

 the lake must have been evaporated to dryness and the resulting salt beds were buried under 

 inwashed detritus; and the lake of the later epoch must have contained fresh water because it 

 failed to redissolve the buried salts of its predecessor and because its own stages of no overflow 

 were so short compared to its stage of overflow that its waters could not become saline. It 

 may be interpolated that good support for the possibility of this group of assumptions was 

 later found by Russell in his studies of the Lahontan area. The assumptions of the second 

 group concerned the effects of lake-water composition on the deposition of inwashed sediments 

 and are that : the processes of sedimentation must have been delayed by the salinity of the earlier 

 Bonneville waters ; and hence that a large share of its inwashed sediments must have been swept 

 offshore into the body of the lake before they settled to the bottom, where they formed the 

 relatively thick lake-floor clays; but that sedimentation must have been accelerated by the 

 fresh water of the second lake, and hence the clays must then have settled nearer the shore and 

 a larger proportion of calcareous material must be found in the relatively thin lake-floor maris. 

 Conversely, the shore deposits of the later lake should show relatively heavier clay deposits 

 than those of the earlier lake; but this consequence of the hypothetical assumptions can not 

 be tested, because of the prevalent coarseness of the deposits near the shore in both the earlier 

 and the later lakes. 



The hypothesis, as thus elaborated, explained very well the facts that it was made to explain, 

 but it did little more; it embodied, however, a group of inferred consequences — the buried salt 

 beds and their shroud of sediments laid down in the arid epoch — the actual occurrence of which 

 would give strong support to the hypothesis if they could be discovered by borings in the lake 

 floor; but they have not yet been discovered. Even the gravels intercalated between the clays 

 and the marls were not found at a less altitude than some 200 feet above the present lake ; and 

 borings later made in the Lahontan sediments failed to discover any purely saline precipitates ; 

 hence it must be inferred that if complete dessication took place, the inwash of muddy detritus 

 during the arid period rendered the precipitates impure beyond the point of identification. 

 Certain parts of the Bonneville history therefore remained unproved. 



It may be well understood that so logical a thinker as Gilbert recognized the unproved 

 elements of his hypothesis, for directly after announcing the above-stated explanation of the 

 clays and marls he added: 



On the whole, the theory that the lake became fresh by desiccation finds too little positive support to entitle 

 it to unreserved acceptance, but it is contradicted by no single known fact, and may therefore be considered 

 to hold the position of a plausible working hypothesis. 7 



It is, indeed, not improbable that many readers of the Bonneville monograph came to have 

 a greater measure of confidence in this "plausible" chapter of the lake history than Gilbert 

 had himself; for when he finally summarized "the general history of Bonneville oscillations" 

 and confidently announced the occurrence of the yellow clays and the white marls of the two 

 lacustrine epochs as well as the complete desiccation of the lake waters during the intermediate 

 arid epoch, he added that he was "practically without information" as to the degree of des- 

 iccation attained in the arid epoch, and he qualified the explanation by which the clays and the 

 marls were differentiated with the halting phrase: "If it be true that the water was so con- 

 stituted . . ." 8 But in spite of these guarded phrases, a computation, which "under this 

 postulate indicates that the first high-water epoch was not less than five times as long as the 

 second," was given graphic representation by a curve; and it is very probable that the un- 

 qualified character of such representation, taken with the ease of apprehending its intended 



I Contributions to tho history of Lake Bonneville. 2d Ann. Rep. U. S. Geol. Survey, 1881, 169-200; see p. 180. 

 « Monogr. 1. 1S90, 260, 261. 



