88 HOT SPRINGS, ARKANSAS. 



HOT SPRING TUFA DEPOSIT. 



As already noted, the hot-spring area is characterized by a deposit 

 of calcareous tufa, or travertine, formed by the hot waters, and cover- 

 ing- not only a large part of the mountain slope about the existing hot 

 springs, but also extending westward to the Happy Hollow ravine and 

 occurring far above any existing springs in the slope above the band 

 stand (Plate III). Tufa deposits are common about both hot and cold 

 water springs whose waters carry carbonate of lime in solution. This 

 material is precipitated when the carbon dioxide of the waters escapes 

 upon exposure of the water to the atmosphere. At the Arkansas Hot 

 Springs a very small amount of carbonate of lime is held in the waters, 

 yet it is sufficient to coat the hot- water pipes and to fill, wooden troughs 

 used to conduct the waters. In the Cave spring and at the Dripping 

 spring (Plate VI) the tufa may be seen now forming. It is therefore 

 not certain that the waters which formed the great tufa deposits of the 

 place were any richer in lime carbonates than those of to-day. This 

 tufa is seen in its natural state at many places about the springs, but is 

 particularly well seen at the Cave spring back of the Arlington Hotel. 

 It is of a gray color and porous texture on the surface, but when quar- 

 ried is pure white, compact, and crystalline. The steps shown in Plate 

 VII are cut in it and the porous texture is seen in the fragments on 

 edge alongside. 



This tufa consists almost wholly of carbonate of lime, carrying very 

 small and varying amounts of manganese (oxide) and iron oxide. The 

 manganese is frequently prominent as a black powder, or occurs in 

 blackish layers through the rock. The analysis made for Owen in 1859 

 of the material deposited in the pipe accords so exactty with that of the 

 deposit now forming that it is reproduced. 



Analysis of hot-spring tufa formed in pipes carrying hot water to bath houses. 



Per cent. 



Carbonate of lime 92. 620 



Sulphate of lime 085 



Carbonate of magnesia ' 3. 060 



Carbonate of iron 210 



Carbonate of manganese 190 



Potassa 107 



Silica 119 



Total 1 99. 391 



In the Cave spring the f reshty deposited tufa is tinted orange by the 

 alga? that live in hot water, and green by the species that flourish at 

 slightly lower temperatures. These colors are purely vegetable and 

 disappear if the deposit be heated. 



This tufa deposit covers an area of approximately 20 acres, and varies 

 from a few inches to 6 or 8 feet in thickness. Its occurrence shows 

 that some of the springs formerly flowed to the west, and that the 

 waters covered a larger area than at present. 



The broad area covered by the tufa does not mean that the hot 

 waters covered this entire area at any one time, for the algous growth 

 described as filling the hot-water streams causes a filling up of the 

 channel and a diversion of the water to a different place. In two 

 instances the waters built up mounds about the springs. The most 

 noticeable of these is that of the Cave spring, which has been artifici- 



