CONDITIONS UNDEK WHICH TUFA WAS DEPOSITED. 211 



the ancient tufa was deposited in a similar manner and was a direct precip- 

 itate from lake waters. It is evidently not a pseudomorph, and since its 

 deposition has undergone but slight change. 



We have already noted that the dendritic tvifa is much moi-e closely 

 related in its structure to the lithoid than it is to the thinolitic variety. The 

 alternation of lithoid and dendritic tufa in narrow bands indicates that the 

 conditions under which they were deposited were very similar. At the time 

 each of these varieties was precipitated the lake was of broader extent and 

 had a much greater depth than when the crystallization of the thinolite 

 took place. From these facts we conclude that the dendritic tufa, like the 

 lithoid, was precipitated when the lake waters were moderately concen- 

 trated At the time of its formation, however, they must have been more 

 highly charged with chloride of sodium, alkaline carbonates, etc., than 

 during the early part of its history. The presence of these salts may ac- 

 count for the peculiar forms assumed by the calcium carbonate upon crys- 

 tallizing. This variety of tufa, like the lithoid, has remained practically 

 unchanged since it was deposited, and cannot be considered a pseudomorph 

 after any other mineral. 



From the relative height of the various tufa dejjosits on the sides of 

 the Lahontan basin we know that the lake was much lower during the thi- 

 nolitic stage than when the other varieties of tufa were formed. It was, 

 therefore, presumably a more concentrated chemical solution. This state- 

 ment i-equires qualification, however, when we consider that between the 

 lithoid and thinolitic stages the lake sank far below the thinolitic terrace and 

 may have undergone complete desiccation. If the lake was evaporated to 

 dryness at that time, one of three results might have ensued : (a) The pre- 

 cipitated salts might have been buried beneath playa deposits, in which case 

 the lake formed when the basin was partially refilled might have been essen- 

 tially fresh, (b) The lake might have been partially refilled before any of 

 the precipitated salts were buried, in which case it would be an alkaline and 

 saline solution of the same character as during the low stages previous to 

 desiccation, (c) Lastly, a partial precipitation and burial of the saline con- 

 tent might have occurred; in this case the less soluble salts would have been 



