CONDITIONS OF SEDIMENTARY DEPOSITION. 5 I 5 



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 

 lithifyingat the point of, and immediately upon, separation. Gil- 

 bert 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 forma- 

 tion of the coherent and incoherent deposits of the same mater- 

 ial from the same water he suggests that "separation was pro- 

 moted by aeration 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 under- 

 current 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- 



1 "Geological History of Lake Lahontan," p. 190. 



