72 ST. LOUIS GROUP. 



§ 151. These underground avenues have resulted from percolating water, 

 without the intervention of earthquakes or other extraordinary agency. Surface 

 water from ordinary rain-storms, finding its way through the ground as it does, to 

 supply common springs, will take up carbonate of lime in chemical solution in 

 limestone countries, and by so doing the fissures through which it passes will he 

 enlarged. In massive limestones with thin, shaly partings, the constant action for ages 

 of percolating water, aided by disengaged carbonic-acid gas, will enlarge the fissures 

 into rivulets, which will culminate in a subterranean river, finding an outlet in some 

 open stream at a lower level. Such is the process by which the rink-holes, 

 caverns, and subterranean streams in this Group of rocks have been formed. 

 Slight projections on the walls record the different stages of the streams as they 

 were slowly cutting their way to greater depths in the limestone. At the bottom 

 of caverns where little or no water is now flowing, rounded pebbles thai have 

 played their part in grinding out the channels occur, as well as sand and clay. 



§ 152. When water, holding bicarbonate of lime in solution, slowly drops from 

 the ceiling of a cavern, exposed to the air long enough to allow one equivalent of 

 carbonic-acid gas to escape, the lime is crystallized. If the deposit take- place 

 from above downward, in the form of an icicle, it constitutes stalactite ; but if it forma 

 on the floor, from below upward, it isstalagmite. These two sometimes meet and 

 form columns. If the solution which forms the stalactites is free from oxide of 

 iron and other impurities, they will be translucent or milk-white. The presence 

 of iron gives them a dirty yellow, red, or brown color. The chambers in which 

 gypsum occurs are dry, and when rosettes of alabaster or translucent lime are 

 formed the caverns must be dry, as they will not form in a damp atmosphere. 



§ 153. The fossils having the greatest distribution, and which are most char- 

 acteristic of this Group are IAAottntitm canadeim, />. proUferum, Producing ovafca, 

 P. marginicinrtus, Mefonites multiporux, Myal'ma d liuiovici, TemnocJieilus coxanum, and 

 Solenocfieilus coUectum. Ores of lead ami zinc occur in pockets and fissures in 

 Livingston, Crittenden, and Caldwell Counties, Kentucky, and at Rosiolare, Illinois. 

 The ores are associated with floor spar and calc spar. The principal gaugue with 

 which the lead is associated in Hardin County, Illinois, is fluor spar, and it is 

 thoroughly disseminated through it. The fluor spar is used for the manufacture 

 of hydro-fluoric acid, and as a flux for smelting ores, where sulphuret of zinc is 

 associated with galena. Lead occurs associated with different minerals and in 

 many Groups of rocks, but never appears to have had an igneous origin. 



