338 SYDNEY H. BALL 



1. High carbon-dioxide content. — The limestone deposition of 

 the Mississippian period, as ChamberHn has shown, set free a large 

 amount of carbon dioxide. This carbon dioxide acted as a blanket 

 to keep in the sun's heat, and greatly increased the power of water 

 to dissolve limestone. 



2. High barometric pressure. — Carbon dioxide being over one 

 and one-half times as heavy as air, its presence would increase the 

 barometric pressure, thus proportionately increasing the chemical 

 activity of gases and liquids. 



3. High and constant temperature. — In post-Burlington times the 

 Ozark region was an island about 160-200 miles in diameter. Because 

 of its insular position and the high carbon-dioxide content in the 

 atmosphere, the climate must have been humid, equable, and warm. 

 The plants of the Coal-Measure shale indicate a tropical or sub- 

 ropical climate. The chemical activity of water at 0° C. is almost 

 nil, but with increase in temperature the solvent power increases out 

 of all proportion to the thermal rise. Since seasonal changes were 

 doubtless slight, solution could work the year around. 



4. High humidity. — The seas surrounding the Ozark island 

 would furnish abundant moisture both to abstract the carbon diox- 

 ide from the air and to bear it through the rocks. 



5. Soluble material to act on. — The purity of Burlington lime- 

 stone and the porosity of the Cambro- Silurian dolomites render each 

 easily soluble, and caves, sinks, and natural bridges are common in 

 the Ozark region today. As the Burlington limestone is coarse and 

 the Cambro-Silurian dolomites medium-grained, the relative solu- 

 bility of the latter would be increased. As an illustration of this the 

 crinoids, the largest calcite grains in the Burlington hmestone, are 

 the last to be silicified in the metamorphism to chert. 



6. Moderate topographic relief. — Moderate topographic relief fur- 

 nishes conditions favorable to slow percolation and unfavorable to 

 excessive mechanical erosion. The presence of post-Burlington 

 sink-holes on the upland and in the adjacent valleys indicates that 

 the level of post-Burlington ground water, and in consequence the 

 topography, was broadly similar to that of the present day. 



7. Thick mantle rock. — Thick mantle rock tends to retard run- 

 off and to increase the time in which solvent waters may act. Solu- 



