234 C. R. KEYES CRUSTAL ADJUSTMENT IN MIfc?SISSIPPI VALLEY. 



this very reason, the stratigraphic importance of the formation has been 

 greatly overestimated. It has led to the attachment of far too much 

 significance to really trivial characters, which, though they may be quite 

 conspicuous in themselves, are of comparatively small value. Features 

 which in other formations would be entirely overlooked, in connection 

 with coal seams become greatly magnified on account of their bearing 

 upon the expense of mining. Among these factors may be mentioned 

 the various kinds of folds or flexures, faults, slips and " cut-outs." 



Folds. 



Principal Features. — Though composed of flat lying beds as a rule, the 

 strata of the Continental interior nevertheless present evidences of oro- 

 graphic movements, though they may perhaps in most cases be slight. 

 Probably the most apparent expression of this action is shown in a series 

 of low folds the general trend of which is north and south. The most 

 prominent of these great corrugations are five in number. 



In the extreme east of the region there are the most westerly anticlines 

 of the Appalachian system of mountains with its closely appressed folds 

 running southwestward from New England to central Alabama. Next 

 is a broad dome-like elevation which finds expression in the uplift of 

 central Tennessee, the Cincinnati arch and the minor elevations of the 

 older rocks in northern Ohio and western Ontario. The axis of this fold 

 extends from lake Huron southward, with a little inclination to the 

 west. Midway between the two great mountain chains of America is a 

 third slight fold whose anticlinal axis extends approximately along the 

 line of the Mississippi river. It is shown in the rocks of central Arkan- 

 sas, in the eastern part of the Ozark uplift, in the many exposures of 

 strata older than the Carboniferous in northeastern Missouri and eastern 

 Iowa, in the " Isle of Wisconsin," and in some of the ancient crj^stallines 

 of the lake Superior region. The outcrops of the older Paleozoic rocks 

 along the Mississippi river cannot be regarded as due entirely to unaided 

 erosion. Apparently the deep gorges of the great river are due partly 

 to the results of the ordinary action of running water ; partly to the result 

 of an accelerated erosion on account of the gradual elevation of the 

 principal line of drainage.^ There is evidence at hand to show that the 

 movement, slight as it may have been, had already begun before the close 

 of the Lower Carboniferous in the present upper Mississippi valley. 

 The fourth fold is perhaps somewhat imperfectly defined at present, but 

 it is indicated by a line of small areas of very ancient rocks trending 

 northAvesterly through central Texas and Indian territory, and protrud- 

 ing through much younger strata. The last is a series of deformations 



