SUB-LAOUSTEAL SPRING DEPOSITS. 221 



and calcium carbonate, upon mingling with tlie waters of a lake may part 

 with their dissolved gases and deposit calcareous tufa. Again, the waters 

 of a lake may be such a strong brine that calcium carbonate cannot be 

 retained in solution, as is the case with Great Salt Lake at the present time. 

 In such an instance the calcium carbonate contributed by springs would be 

 precipitated when their waters mingled with those of the lake. Phenomena 

 of this nature have been observed at the Needles, as described on page HI, 

 and may be studied at a number of localities in Mono Lake. This lake, as 

 shown by the analyses given on table C, is a strong solution of the car- 

 bonate and the sulphate of soda, chloride of sodium, etc., while many of the 

 springs that rise in it are quite remarkable for their purity, but yet contain 

 a small percentage of calcium carbonate wliich is deposited about their 

 points of discharge. These accunmlations frequently form domes of large 

 size that are p'orous and tubular in structure, and in many respects resemble 

 those standing in the deserts of the Lahontan basin. Li numerous instances 

 the deposits from tlie springs in Mono Lake form irregular tubes that are 

 clustered together and frequently branch and expand as they grow upwards, 

 thus forming columnar and vase-shaped structures. A most instructive 

 exhibit of this nature is to be seen in the western portion of Mono Lake, near 

 the mouth of Mill Creek. At this locality, a part of which is shown in 

 Plate XLIII, there are as many as fifty or sixty tufa domes standing in 

 from twelve to fifteen feet of water, many of which rise from ten to twelve 

 feet above the surface of the lake. The to])s of some of these structures 

 are occupied by basin-shaped depressions, which in a few instances, are 

 filled with water that rises through the irregular tubes and open spaces in 

 the column beneath. The water in these basins is cool and fresh, and over- 

 flowing, fountain-like, down the sides of the vases, mingles with the waters 

 of the lake. These structures are still growing by the gradual 'precipitation 

 of calcium carbonate, which is taking place, however, only above the lake 

 surface. They are nearly always considerably smaller at the lake surface 

 than at the top, and in general form are not unlike the sponges known as 

 Neptune's cups, found in southern seas They are not only striking 

 examples of chemicall}' formed rocks that ai-e of interest to the geologist, 

 but they are fountains of sweet water in the midst of a lake that is utterly 



