December 4, 1914] 



SCIENCE 



829 



until at tlie higher levels it merges with the 

 lithoid tufa. In other words, the gradation be- 

 tween the lithoid and dendritic types in the 

 Lahontan basin is a vertical one and was ap- 

 parently caused by the increasing calcium car- 

 bonate content of the receding lake waters. 



Russell placed the upper limit of the thino- 

 lite tufa at the thinolite terrace now one hun- 

 dred feet above the lake. During the present 

 study it was found that the thinolite extended 

 some forty feet above the terrace throughout 

 the Pyramid Basin and there ended in a few 

 scattered crystals at the base of the dendritic 

 tufa. Dana* after a careful study of the thin- 

 olite crystals came to the conclusion that it 

 was a pseudomorph after an unknown tetra- 

 gonal mineral, possibly a chlor-carbonate of 

 calcium. Measurements by the vn-iter of some 

 of the more perfect crystals lead to the con- 

 elusion that the original mineral was ortho- 

 rhombic and probably aragonite. On experi- 

 mentation it was found that when a saturated 

 solution of calcium carbonate was added to the 

 water from Pyramid Lake minute crystals of 

 aragonite closely resembling in their detail 

 the smaller crystals of the thinolite separated 

 after standing a day or two. The thinolite, 

 therefore, was deposited as aragonite at a time 

 when the waters of Lahontan were saturated 

 with calcium carbonate. 



As was recognized by Russell, the interme- 

 diary gravels are bar and beach deposits, and 

 as only the lithoid tufa is associated with them 

 they represent a low water stage in the filling 

 of the basin. . 



To sum up the present evidence the broader 

 features of the history of Lake Lahontan are 

 believed to be as follows. As the Lahontan 

 Basin began to fill the waters approximated 

 those in the present rivers flowing into it and 

 contained but little calcium carbonate. As a 

 consequence the algse as they became estab- 

 lished on the rocks about the lake were able to 

 deposit only the lithoid tufa. With the climax 

 of the lake when evaporation became the con- 

 trolling factor and the lake diminished the 

 relative amount of calcium carbonate present 



*Dana, E S., Bull. No. 12, U. S. Geol. Surv., 

 1884. 



increased more rapidly than the algse could re- 

 move it. With the increasing amount of cal- 

 cium available the ts^pe of tufa deposited 

 gradually changed to the dendritic. Neverthe- 

 less, the calcium continued to increase until / 

 when the lake had fallen three hundred and 

 fifty feet from its highest level, the waters 

 were saturated and the surplus was deposited 

 as aragonite forming the thinolite tufa. With 

 the removal of the surplus calcium and prob- 

 ably aided by the slower fall of the lakes, ow- 

 ing to the diminished area the algse caught up 

 and have removed the calcium faster than it 

 was brought in by the rivers until at the pres- 

 ent time the lake contains only one twentieth 

 the amount of calcium possible and only the 

 lithoid tufa is being deposited. 



As there is little or no evidence of an un- 

 conformity in the tufas, they in themselves do 

 not indicate more than one lake period. The 

 intermediary gravels are all above the pres- 

 ent level of the lakes and from such evidence 

 as has been gathered represent but a low 

 water stage in the early filling of the lake 

 basin. With the conclusion that Lake Lahon- 

 tan had but a single period of extreme high 

 water the question naturally arises as to 

 whether the lake has completely disappeared. 



One unfamiliar with the area and reading 

 the reports of King and Russell is apt to 

 gather the impression that a large part of the 

 basin is covered by the tufa. This is not cor- 

 rect. As was noted by Russell, the tufa is ex- 

 tremely localized and occurs for the most part 

 in large isolated masses. It is only the nar- 

 row band of recent tufa about the present 

 shores of Pyramid Lake that approaches a 

 continuous layer and it probably covers less 

 than fifty per cent, of the shores exposed. A 

 liberal estimate of the tufa-covered area of 

 the basins of Pyramid and Winnemucca lakes 

 is one per cent, of the total area of these 

 basins and the tufa is much more abundant 

 here than in the remainder of the Lahontan 

 Basin. Estimating an average thickness of 

 eight feet of tufa belowi the Thinolite terrace 

 and three feet above it to the high-water mark 

 gives sixty-five million tons of tufa deposited 

 in the basins of the two lakes in question. 



