110 LAKES OF NORTH AMERICA. 



t 



The precipitation of calcium carbonate from lake Avaters takes place 

 principally in two ways ; it may separate in the open lake and fall to the 

 bottom in a finely divided state and become mingled with mechanical 

 sediments so as to form marls, or it may be precipitated where solid 

 rocks occur and cover them with a dense incrustation. The ability of 

 ordinary surface waters to dissolve calcium carbonate, depends mainly on 

 the carbonic acid gas they hold in solution. Lake waters lose their dis- 

 solved gases most rapidly where they form breakers along the shore, as in 

 such instances they are most thoroughly aerated. For this reason, the 

 boldest headlands are apt to receive the heaviest deposits of tufa when the 

 waters dashed against them became concentrated. It is at such localities 

 that the principal deposits of tufa in the Bonneville basin occur. It 

 happens also that calcium carbonate has a tendency to accumulate about 

 solid l)odies, not only because they afford a stable support, but for the 

 additional reason that points and angles induce crj-stallization. Calcareous 

 tufa was deposited in vast quantities about the shores of Lake Lahontan 

 wherever there were rocky slopes and in increasing abundance from an 

 horizon high up on its borders down to the deepest point now exposed. 

 The fluctuations of level in Lake Bonneville were recorded principally by 

 beaches and embankments of mechanical origin ; similar changes in Lake 

 Lahontan are made known by tufa deposits of chemical origin. 



The tufa of the Lahontan basin presents three main varieties, each of 

 which is composed of concentric layers as is shown in Plate 21. The 

 smaller divisions seem to indicate minor changes in the chemistry, and 

 perhaps also fluctuations in the temperature, of the water from which they 

 were precipitated. The three principal varieties have been named in the 

 order of their formation, Lithoid, Thinolitic, and Dendritic tufa. Lithoid 

 tufa is a compact stony substance with a granular te:?fture ; Dendritic tufa 

 has an open structure and resembles a mass of branching twigs turned to 

 stone ; and Thinolitic tufa, shown in Plate 22, is composed of well 

 defined crystals to Avhich the name Thinolite was given by Clarence 

 Kino-. The composition of each of these varieties is the same. They are 

 composed of calcium carbonate with usually some slight amount of im- 

 purities. Their wide variation in structure and general appearance, is 

 due to differences in the condition of tlie lake waters at the time of 

 their formation. 



About Pyramid lake, where the Lahontan tufas are usuall^r . well dis- 

 played, the first or Lithoid variety reaches a height of 500 feet, the Thino- 

 litic 110 feet, and the third or Dendritic variety, 320 feet above the 



