STUDIES OF SPECIAL LACUSTIIAL IIlSToKV. 109 



the reason thai it was the hirovr of the two Avater l)()dies and had a more 

 reguhir outline, thus giving the wind a better opportunity to act on its 

 waters, and also because it was held at a detinite level for a long period, 

 or rose to the same hoi'izon at various times, on account of its having an 

 otitlet. 



The hio-hest water line al)out Lake JJonneville. named the "Bonneville 

 beach,'' is conspicuous not so much on account of its strength as for the 

 reason that it marks the dividing line betv.een rain sculpture on the 

 higher portions of the bordering mountains and the characteristic topogra- 

 phy due to the work of waves and currents on their lower slopes. Tlie 

 channel of discharge was lowered until a sill of resistant limestone was 

 reaehed whieh determined the horizon of the strongest and best developed 

 terraces and embankments in the basin. A well defined beach at this 

 liorizon is known as the " Provo beach." the name being derived from the 

 town of Provo, Utah, which stands on a broad delta formed by the sedi- 

 ment of Provo river, when the lake stood at the horizon of its lowest 

 point of discharge. The wave-built structures marking the Prove stage 

 are on a magnificent scale and are still almost as fresh in appearance and 

 })erfect in form as if abandoned by the waves but yesterday. In the 

 Lahontan basin the shore topograph}' w^as never strongly pronouneed. 

 Fluctuations of level Avere not controlled by an outlet, and the numerous 

 islands and headlands diminished the influence of the wind and checked 

 the action of Avaves and currents. 



The chemical histories of lakes Bonneville and Lahontan are fully as 

 instructive and of as great interest as their physical changes. In this 

 connection, the l)asin of Lake Lahontan has been found to exceed its 

 companion in the completeness of its records. The eseape of tlie waters 

 of Lake Bonneville insured its freshness during a part of its history. The 

 absence of an outlet for the waters of Lake Lahontan led to a high degree 

 of concentration. 



When lake waters are concentrated by cA'aporation the first substance 

 to be precipitated, as previously described, is ealcium carl)onate. About 

 the shores of Lake Bonneville there are in favorable localities, consider- 

 able deposits of tliis sul)stance in the form of coral-like inerustations 

 knoAvn as calcareous tufa. It appears on rocky points and forms a cement 

 for gravel and sand on the outer borders of some of the terraces, but is 

 insignificant in amount and simiile in character, AA-hen compared with 

 the trul)' immense accumulations of a similar nature in the Lahontan 

 basin. 



