LIME PRECIPITATES IN LAKES 



339 



of hydrogenic lime are recognized by Russell (47:^//; 48) in the 

 basin of Lake Lahontan, where they covered the surfaces of nearly 

 all the rocks, on the shores and bottom of the old lake. The oldest 

 deposit is the Litlioid tufa. It is compact, gray, sometimes form- 

 ing a cement for the ancient gravels, and not infrequently contain- 



FlG. 64. A group of crystals of thinolite, or thinolithic tufa. Lake Lahontan 

 basin, Nevada. (After Russell.) 



ing the shells of fresh-water gastropods. After an interval of ex- 

 posure the second type, or ThinolitJiic tufa, was deposited over the 

 first. It consists of orthorhombic prisms 6 to 8 inches in length 

 and almost half an inch thick (Fig. 64). These have a peculiar in- 

 ternal structure, and were at first considered pseudomorphs after 

 gaylussite, but their structure does not agree with that of any 

 known mineral. The entire layer is from 6 to 8 feet thick where 



