46 IOWA ACADEMY OF SCIENCES. 



occasional out-croppings of the same at the base and on the slope at variable 

 altitudes. The topography of this country is rather simple and in a way 

 monotonous, yet somewhat difficult to map on account of the numerous 

 protruding spurs or points. The contours appear near the base rather far 

 apart; toward the summit they run closer together and at nearly regular 

 intervals until the topmost contour is reached, when a break in the regu- 

 larity is occasioned. Hei-e when the interval is as great as twenty feet the 

 line frequently extends perhaps a mile or more with little curvature, both 

 sides parallel with the axis of the ridge. When these chert-covered areas 

 are more limited in extent hills rise as individual points separated by 

 syncliues whose troughs are of nearly the same level. In this case the reg- 

 ular contouring is unbroken from base to summit. 



The other limestone regions are recognized by. gently sloping fields cap- 

 ping the heavily bedded stone which, upon decomposition gives rise to the 

 dark red coloring prevalent in the soil above and the associated clay. The 

 larger streams traversing a region of this character leave banks of roughly 

 weathered rock and but little detrital material; along the smaller ones bed- 

 ded rock is seldom exposed. 



There is presented in a region distinctly of sandstone a barren, for the 

 greater part level, ai'ea cut by ravines, one or both banks being of solid 

 sandrock ofttimes left in ovei'hangiug ledges by the eating out of the under 

 portions of the bed. Along the nearly level tops, outcrops of sandstone are 

 frequent, as the overhanging soily material, which in time accumulates 

 does not foi'm a mantle of equal thickness over the surface of the stone, but 

 being of a coarse, sandy nature, is transferred from place to place and col- 

 lected in heaps leaving other portions bare. 



Over southern Missouri the limestones and sandstones are so closely, 

 albeit irregulai'ly, associated that their mode of occurrence, extent and rel- 

 ative position in the geological scale has long been a subject of discussion. 

 Here there are extensive i-egions where both kinds of rocks prevail. Out- 

 crops are common; first one then the other appear, over-lapping and inter- 

 locking. The problem of classification becomes intricate. The general sur- 

 face features of a combination of the two sedimentaries are not like those 

 of either alone. However, the type exhibited is but little more than a com- 

 bination of the types represented by the rocks separately. 



The topography of the Saint Francois mountains, this name having been 

 applied by Winslow to the porphyry and granite hills and mountains of the 

 region in question, is indeed striking, much more so than is that of the 

 Ozarks to the west. 



These Archasan mountains stand out in bold relief among the knobs of 

 the Cambrian. Where both occur the beauty of the relief maps is greatly 

 enhanced by the presence of narrow gorges and steep acclivities which 

 not uncommonly form solitary high peaks of conical form, which are sur- 

 rounded on all sides by lower lands of the Cambrian. 



Porphyry makes up by far the greater portion of the 400 square miles of 

 crystallines. The style of topography although varying is impressive, and 

 in not a few instances at all similar to that of the granite. But as a whole, 

 the type which represents either rock will almost invariably prove itself 

 strictly characteristic of the ancient rocks. The nearest approach to the 

 porphyry type is that of the ch6rt-capped limestone hills previously men- 



