ANALYSES OF TUFAS. 



203 



clays. The occurrence of evenly stratified lacustral beds, containing the 

 shells of fresh-water mollusks, above the layer of dendritic tufa, is evidence 

 that the lake rose after the precipitation of the tufa and was essentially 

 fresh. 



As in the earlier formed varieties, a section of the sheathing of dendritic 

 tufa exhibits many alternating bands, showing that the deposition took 

 place from without and was subject to many variations. 



CHEMICAL COMPOSITIOK OF THE TUFA DEPOSITS. 



Carefully collected samples of each of the varieties of tufa described 

 above were submitted to Prof. O. D. Allen, of Yale College, for analysis, 

 who reports their composition as follows : 



Constituents. 



Insoluble i esidue 



Lime (CaO) 



Magnesia (MgO) 



Oside of iron and alumina . 



Carbonic acid (CO?) 



■Water (HaO) 



Phosphoric acid (POs) 



Chlorine and sulphuric acid 



Totid 



Llthuid tufa. 



1.70 



60.48 



2.88 



.25 



41.85 



2.07 



.30 



trace. 



99.53 



Thinolitic 

 tufa. 



Dendritic 

 tufa. 



3.88 



50.45 



1.37 



.71 



40.00 



1.50 



trace. 



trace. 



5.06 



49.14 



1.99 



1.29 



"" 40. 31 



2.01 



trace. 



trace. 



98.81 



It will be seen from this report that the composition of the tufas gives 

 no hint as to differences in the conditions under which they were deposited. 

 With the exception of the insoluble residue, which ma}^ be considered in a 

 measure as accidental — being in part foreign matter imprisoned during the 

 precipitation of the tufa, and in part carried into the insterstices of the rock 

 as atmospheric dust after the desiccation of the basin — the various specimens 

 have essentially the same composition. Special tests have been made in 

 the case of thinolite to determine if in some instances it might not contain 

 notable quantities of calcium chloride or other similar salt, but the results 

 were negative. In common with all the tufa deposited in the Quaternary 

 lake basins of the Far West, the several Lahontan varieties, as found at the 

 present day, are simply impure calcium carbonate. 



