186 GEOLOGY OF OHIO. 



of the county and of visitors from other States, and deserve a few words 

 of description and explanation. As is known to most persons, at Cas- 

 talia a volume of water which forms quite a river flows up from several 

 deep orifices in the limestone rock, and supplies in its descent to the 

 Lake the motive power for several mills. The water maintains nearly 

 the same temperature winter and summer, and its flow is more uniform 

 than that of surface streams in the vicinity, though sensibly affected 

 by periods of unusual and wide-spread drouth. The water of the springs 

 is highly charged with lime, rapidly incrusting any object covered by 

 it, and it has deposited a sheet of travertine over an area of several 

 square miles in the vicinity. The rock in which the subterranean chan- 

 nels are excavated, through which the waters of the springs flow, is the 

 Waterlime, the uppermost member of the Silurian system. This is a 

 magnesian limestone, in fact, a typical dolomite, containing about forty- 

 two per cent, of carbonate of magnesia and fifty-five of carbonate of lime. 

 This rock forms on the surface an unbroken sheet, reaching from Cas- 

 talia to Logan county, the highest land in the State. The true theory 

 of the formation of these springs is simply this : the Helderberg lime^ 

 stone, like many others, is soluble in atmospheric water containing 

 carbonic acid It forms the slope of the watershed, and the drainage 

 of the country south from Castalia, passing over and through it, has 

 dissolved out a connecting system of channels which are really subter- 

 ranean rivers. Castalia Springs are formed at the mouth of one of 

 these. Similar springs and underground streams are met with in all 

 limestone countries. The table-land of central Kentucky affords innu- 

 merable examples of them. This plateau is underlain by a thick mass 

 of unusually soluble limestone. The surface water dissolves it away so 

 easily that it enlarges every crack it penetrates, and has formed a con- 

 nected system it underground channels by which, all the drainage of 

 the country is effected. The celebrated Mammoth Cave is only one of 

 these channels. Along the margin of this plateau there are a great 

 number of fountains like Castalia Springs, which mark the mouths of 

 the subterranean streams that have been described. Such fountains are 

 also common in other countries, and the classical Clitumnus bursts out 

 at the foot of a limestone mountain; forming a. fountain precisely like 

 that of Castalia. 



