828 



SCIENCE 



[N. S. Vol. XL. No. 1040 



indicated otherwise. Finding the lakes at 

 present occupying portions of the basin to be 

 comparatively fresh, he believed that Lahon- 

 tan had been completely desiccated and its sa- 

 line deposits buried before the present lakes 

 formed. 



EusseU recognized three types of the tufa 

 in the Lahontan Basin ; the lithoid, stony and 

 compact; the dendritic, porous and coralline; 

 and the thinolite, crystallized and a pseudo- 

 morph after an unknown mineral. For a dis- 

 tance of one hundred feet above the present 

 level of Pyramid Lake these were found to be 

 superimposed, the lithoid at the base, suc- 

 ceeded by the thinolite, and the dendritic cov- 

 ering the other two. While he recognized that 

 there was some lithoid tufa intermingled with 

 the dendritic, he believed that the lithoid had 

 been deposited from waters of slightly differ- 

 ent composition from those forming the den- 

 dritic tufa. Finding a medial series of grav- 

 els in the lake sediments, he believed that La- 

 hontan had formed, diminished through desic- 

 cation and deposited the lithoid tufa, had 

 again risen nearly to its former level, had 

 again been desiccated, depositing the thinolite 

 and dendritic tufas and, completely disappear- 

 ing, left the other salts to be buried beneath 

 the alluvium that swept in from the surround- 

 ing moim.tains. Later a slight change to a 

 more humid climate produced the present 

 lakes. 



In the course of work in the vicinity of the 

 Salton Sea,^ done in connection with Dr. D. 

 T. MacDougal, director of the desert labora- 

 tory of the Carnegie Institution of Washing- 

 ton, it was concluded that the tufa at present 

 forming in the sea were being deposited 

 through the activities of blue-green algse and 

 the bacteria associated with them. The more 

 significant evidence discovered was the con- 

 stant association of the algae and the tufa, the 

 extreme localization of the tufas, the forma- 

 tion of the tufa from water that contained 

 considerably less calcium carbonate than would 

 saturate it, and the evident relation between 

 the development of the tufa and the light ex- 



3 Jones, J. C, Pub. No. 193, Carnegie Inst, of 

 Wash., pp. 79-83, 1914. 



posure. Further, a gradual gradation between 

 the dendritic and the lithoid tufas was found 

 in the older tufas of the basin, the lithoid tufa 

 being formed in the dark portions of recesses 

 in the cliffs where the conditions were unfav- 

 orable for the normal growth of the algse. 

 Thin sections of the older tufa show abundant 

 remains of the algse included in the tufa as 

 are the algse in the tufa forming at present. 



Through the kindness of Dr. MacDougal it 

 was possible to immediately carry the study in 

 to the Lahontan Basin. Around the shores of 

 Pyramid Lake a thin coating of lithoid tufa 

 was found coating the rocks and in places ce- 

 menting the gravels. The same close associa- 

 tion between the algse and the tufa was ob- 

 served, and wherever the algae were found the 

 gravels were cemented and where they were 

 absent no trace of the tufa could be found. 

 Thin sections of the older dendritic tufa dis- 

 closed forms similar to those found in the 

 tufas of the Salton Basin and sections of the 

 tufa forming at present showed the algse im- 

 bedded in it. As analyses of the lake water 

 showed it to contain only about one twentieth 

 the calcium carbonate necessary to saturate it, 

 it is evident that the tufa forming to-day is 

 being deposited through the activities of the 

 plant life. As the algse in the Salton Sea are 

 depositing dendritic tufa from waters contain- 

 ing ten times more calcium carbonate than 

 Pyramid Lake and the tufa being deposited in 

 the latter at present is lithoid in type, it is 

 probable that the calcium content of the lake 

 waters is a determining factor in the type 

 formed. 



In the Lahontan Basin a thin layer of the 

 lithoid tufa is in immediate contact with the 

 rocks on which deposits of tufa were formed. 

 Approximately six inches in thickness near the 

 water's edge it decreases in thickness until at 

 the top it merges with the caliche formed on 

 the basalt that is the predominant rock in the 

 basin, and it is practically impossible to say 

 just where the tufa ends and the caliche be- 

 gins. The dendritic tufa is best developed in 

 a horizontal zone between one and two hun- 

 dred feet above the present lake. It gradually 

 diminishes in thickness and changes character 



