CONDITIONS UNDER WHICH TUFA WAS DEPOSITED. 213 



sition of any of tlie contained salts. The water would thus be more highly 

 charged with saline matter when the lake rose to the Jevel of the thinolite 

 terrace than when it previously stood at that horizon. Under these con- 

 ditions the composition of total salts would remain practically constant, but 

 their percentage in a given quantity of water would be increased. Our 

 ignorance of the influence that the presence of various salts exerts on the 

 character of the calcium carbonate precipitated from saline waters renders 

 it impossible for us to predict that it would diflPer in crystalline form when 

 deposited in strong or weak brines. A partial desiccation would cause a 

 more marked change in the chemistry of the lake than continued concen- 

 tration. We are inclined, therefore, to the belief that the former is the 

 moi'e probable hypothesis of the two. 



The positive element in the problem is that a marked change did take 

 place in the character of the calcium carbonate precipitated, which is proof 

 of an alteration in the chemistry of the lake waters, uialess the question of 

 temperature be considered as of weight in the problem. The assumption 

 that this change was an increase in the percentage of alkaline carbonates 

 in the waters is strengthened by the fact that thinolite has only been found 

 in this country in basins characterized by the presence of these salts, viz, 

 in the Lahontan and Mono basins. In the Bonneville basin, tufa was depos- 

 ited on quite an extensive scale, but it did not assume the form of thinolite. 

 Since that basin last overtlowed its waters haA'e been concentrated until 

 they are strong brines, without producing the conditions necessary for the 

 formation of thinolite. It is evident, therefore, that some element in the 

 chemistry of the lakes on the western border of the Great Basin, in which 

 they differed from their sister lakes to the eastward, determined the crystal- 

 line form of the calcium carbonate precipitated from them. This difference 

 was most probably the greater richness of the western lakes in alkaline 

 carbonates. 



It has already been pointed out that lithoid and dendritic tufa must 

 have been deposited from the same solution under slightly varying condi- 

 tions, for the reason that narrow bands of these varieties alternate with one 

 another. . A similar altern;ition of thinolite and dendritic tufa has been ob- 



