214 GEOLOGICAL HISTOEY OP LAKE LAHONTAN. 



served in many places, which seems evidence that these varieties were de- 

 posited from the same waters under somewhat diverse conditions. A char- 

 acteristic feature of inclosed lakes is their inconstancy of level. It seems 

 evident that the banded character of the tufa deposits may be correlated 

 with fluctuations of the lakes in which they were formed. In the case 

 under consideration, we may reasonably assume that when the watei's were 

 concentrated, thinolite was crystallized ; when they became somewhat dilute, 

 the tufa assumed the dendritic form. Finally the lake rose and remained 

 for a long time above the thinolitic limit and only the dendritic variety was 

 deposited. Our observations lead to the conclusion that the changes from 

 conditions favoring the crystallization of thinolite to those admitting of the 

 formation of dendritic tufa or vice versa, during certain stages of the lake, 

 were very slight. It appears quite probable that the alternation of thin 

 layers of these varieties of tufa may record alternate arid and humid peri- 

 ods. That they are not annual growths, thinolite being formed during the 

 summer and dendritic tufa during the winter, is evident from the fact that 

 the quantity of calcium in even the thinner layers is too great to have been 

 contributed to the basin within a single year. 



Professor Dana's studies have shown that thinolite is a pseudomorph, 

 but what the antecedent mineral may have been still remains an enigma. 

 Geological observations when considered by themselves tend to the hypoth- 

 esis that the waters in which thinolite was formed were charged with sodium 

 carbonate, and that the crystals now represented by the thinolite were a 

 compound of soda and lime, presumably as the double carbonate. Professor 

 Dana has shown, however, that this postulated mineral could not have been 

 gaylussite (as supposed by King) and, in fact, was not any natural or arti- 

 ficial crystal that has been recognized. It appears as if the unknown min- 

 eral must have been produced by a delicate adjustment of the chemical con 

 ditions of lake waters — in which the influence of the mass and of tempera- 

 ture perhaps played an important part — which has not been observed in 

 nature or reproduced in the laboratory. In reference to the chemical nature 

 of the original mineral Professor Dana says : 



" The description of the original crystalline form of the thinolite, so 

 far as it can be made out, is sufficientl}- complete to give an emphatic neg- 



