GENERAL GEOLOGICAL FEATURES OF THE REGION. 365 



Faulting is commonly quite unimportant, the throw being usually not 

 more than a few feet. Some notable exceptions, however, are known. 

 The position and extent of the dislocations are more clearly defined in 

 the stratified rocks than in the crystallines. 



Dikes of basic rock occur rather abundantly. They vary from a few 

 inches to 50 yards or more in width and cut the granites and porphyries 

 alike. Nowhere have they been observed to penetrate the overlying 

 sedimentaries. Their number and wide distribution, the great weight 

 and black color of the rock composing them, and their peculiarities in 

 weathering cause them to attract wide attention. 



The crystalline masses of the region are all older than the sedimentary 

 strata which everywhere rest unconformably upon these rocks. A long 

 period of erosion is represented by this unconformity. Abundant evi- 

 dence shows that when the sedimentary rocks were laid down the diver- 

 sity of surface relief was even greater than at the present time. To this 

 ver}^ marked irregularity in the pre-Cambrian surface may be ascribed 

 many anomalous stratigraphic features which are met with throughout 

 the region. 



The sedimentaries are represented by an extensive succession of mag- 

 nesian limestones with intercalated sandstones. . Almost always the 

 latter immediately cover the pre-Cambrian elevations; they also occupy 

 positions between the calcareous beds. 



The stratified rocks dip away in all directions from the central crystal- 

 line area. Younger and younger strata successively form the surface 

 rocks as the distance increases from the porphyry hills. The Paleozoic 

 is well represented from the Cambrian to the Carboniferous. From Iron 

 mountain to the nearest point on the Mississippi river, a distance of 30 

 miles in a northeasterly direction, the entire sequence from the basal 

 granite through the Cambrian, Ordovician, Silurian, Devonian, and Car- 

 boniferous to the Coal Measures, is passed over. 



The crystalline Rocks. 



types represented. 



During the past few years the crystalline rocks of Missouri have been 

 carefully mapped, and the geographic distribution of the principal varie- 

 ties determined. It is due, however, to the petrographical investigations 

 of Haworth * that the various mineralogical and structural types have 

 been made out. 



*Loc. cit. 



