Page:The Journal of geology (IA journalofgeology11893univers).pdf/89

Dittmar's experiments in decomposing bicarbonate of lime by agitation indicate the nature of the condition afforded by the surf, and it appears that the neutral carbonate is capable of lithifying at the point of, and immediately upon, separation. Gilbert also says that: "Calcareous matter constitutes an important part of the fine sediment of the lake bottom, and this was chiefly or wholly precipitated from solution," and to explain the formation of the coherent and incoherent deposits of the same material from the same water he suggests that "separation was promoted by aëration of the water. All precipitation being initiated at the surface during storms, coalescence at the shore may have resulted from contact at the instant of separation."

Mr. Gilbert states (pp. 178-179), that the concentration of the waters of Lake Bonneville at the Provo stage is not definitely known. The lake had an outlet at the northern end of Cache bay, and the principal tributary, Bear river, emptied into this bay near the outlet. Cache bay was connected with the main body of the lake only by a deep but narrow strait, and it is possible that evaporation from the greater expanse of the lake exceeded the inflow of fresh water into it, while the overflow at the outlet was supplied by Bear river. In that case there would have been circulation through the strait between Cache bay and the main body, an upper current from Cache bay and an undercurrent from the lake. The straits were the scene of peculiarly copious deposition of tufa.

The tufa deposited in Lake Bonneville is of the variety described by Russell as "lithoid tufa,"^[1] "of a compact and stony structure" and he concludes that it was formed when the lake waters were moderately concentrated (pp. 210-222). A limestone of similar structure is now forming on the shores of Florida, where the waves break on the beaches under conditions quite like those which determine the growth of tufa, where the surf dashed against the shores of Lake Bonneville. This rock is deposited in irregular layers, sometimes three or four feet thick, on the quart-