36 GEOLOGICAL FORMATIONS 



Francois river, near Fredericktown in Madison county, is 

 shown above. It may be taken as representative of a very 

 large number of exposures occurring throughout the district. 

 The porphyry is perfectly massive, rather dull reddish in color, 

 with numerous dykes of diabase traversing it in various places. 

 The interior of the mass is perfectly fresh. Over its ancient 

 surface it has manifestly been greatly eroded, and is still cov- 

 ered with fragments and boulders of various sizes. The sand- 

 stones, with planes of false bedding distinctly marked and 

 inclined at a high angle, pitch away from the central porphy- 

 ritic elevations. A short distance from the crystalline masses, 

 the sandstones, by the addition of calcareous material pass 

 rapidly into heavily bedded limestones. This transition takes 

 place both upward and laterally. In the former direction the 

 change is often abrupt. The sand grains become fewer and 

 fewer and more widely separated, until within the space of often 

 three feet the passage from a pure silicious sandrock to a 

 homogeneous limestone is complete. The evidence is plain that 

 the great quartz-porphyry and granite masses of the region were 

 raised above the waters of the great interior sea, and profoundly 

 eroded as well as deeply decomposed in situ. When the hills 

 were gradually depressed below the level of the waters, the 

 wave motion quickly removed all loose material on the sur- 

 face, depositing it again near by along the sides of the old 

 peaks. Sandstones and limestones, with occasional clays, thus 

 filled the old valleys. When from any cause the currents be- 

 came stronger, tongues of sand were deposited far out into 

 the waters, again to be soon covered by limestone. Thus on 

 the sloping, sinking shores sands were laid down. Their sea- 

 ward extent varied greatly at different horizons — sometimes 

 covering the calcareous deposits ; sometimes allowing them- 

 selves to be covered. Thus closely following the ancient land 

 surface, a continuous sandstone may be found, representing 

 several or many horizons. Farther outward, or seaward, sand- 

 stone beds are found intercalated in limestone. This disposi- 

 tion of beds is graphically shown in the accompanying dia- 

 gram. ( Figure 2.) 



