TUFA A DEPOSIT OF WEAK BRINES. 219 



Throughout the Lahontan basin the various deposits of tufa are most 

 abundant on steep I'ocky slopes and on isolated buttes which were formerly 

 submerged. Its exceptional abundance at these localities is due principally 

 to the fact that the rocky surface afforded stable support for the precipitates 

 deposited upon them; and its preservation is insured because precipitous 

 shores are in general only slightly modified by wave action, and are not 

 favorable to sedimentation. The dash of the waves against sea cliffs may 

 also promote precipitation for the reason that the waters are aerated, thus 

 facilitating the escape of carbon dioxide, the presence of which is necessary 

 to the solution of calcium carbonate. Wherever tufa occurs on the surface 

 of lake beds a solid nucleus may nearly always be found about which the 

 calcium cai'bonate was deposited. Pebbles and shells lying on the bottom, 

 or rocky points projecting above the mud, were favorable nuclei around 

 which crystallization took place. About such centei's mushroom-shaped 

 growths were formed like those .shown on Plate XXXVII, or domes and 

 castles of great size were slowly built up. Solid nuclei seem essential for 

 the commencement of these imitative structures, and appear to play the same 

 role as the nuclei in the familiar experiments of crystallizing alum and rock 

 candy. Some of the towers and castles in the Lahontan basin, which contain 

 hundreds and even thousands of tons of tufa, are known to spring from small 

 centers of accretion. When crystallization was once initiated, precipitation 

 appears to have been accelerated and may possibly have been continued in 

 waters that were below the point of saturation. When the crystallization 

 began about a small nucleus at the bottom of the lake the tendenc)^ was to 

 build upwards. Owing to this tendency the tufa deposited in isolated 

 localities assumed the form of domes and towers instead of spreading out 

 laterally and forming sheets or thin flat-topped masses. 



The fact that calcium carbonate cannot remain in solution in concen- 

 trated lake waters, but is precipitated as soon as delivered by tributary 

 streams, indicates that tufa is a deposit of moderately saline waters. In 

 Great Salt Lake little more than a trace of calcium is found, and this prob- 

 ably exists as the sulphate. In Mono Lake about six hundredths of one 

 per cent, has been found by analyses. The dense alkaline waters of the 

 Soda Lakes near Ragtown, and of Abert Lake, Oregon, are free from cal- 



