EESULTS OF THE ACTION OF OEOGEMC FOECBS. 21 



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to permit tliem to take on the normal folds which afford the natural 

 means of shortening in stratified beds. 



Since the shearing planes of the massive rocks grow less evident as we 

 go away from the margin of the basin, and have not been clearly observed 

 beyond the limits which are likely to have been occupied by its deposits, 

 the question arises whether these planes are not a consequence of the 

 movements which must have occurred in the rocks that lay beneath the 

 Carboniferous strata at the time they were folded. That some form of 

 distortion affected these basilar deposits must be assumed, but the precise 

 nature of the movements is not known. 



The type of folding exhibited in the stratified rocks of the basin is 

 clearly that of ordinary synclines and anticlines which have been carried 

 to a rather advanced stage of development. As will be noted from the 

 mapSj the axes of these folds trend nearly north and south in the southern 

 poii:ion of the basin, but in the northern part they incline to the eastward, 

 and in the east attain a position nearly at right angles to the southern folds. 

 This turn of the structural axes seems to be due to the existence of the 

 broad eastern bay, which is a notable feature of the basin. 



Thei^e is another noteworthy feature in the form and distribution of 

 the anticlines which appears to be closely related to the peculiar history 

 of this basin; this is clearly exhibited in the accompanying diagrammatic 

 section (fig. 2, p- 27), which shows the attitudes of the folds in the central 

 part of the field along a line from north to south. In this section the 

 compressive action has operated to create strong folds next the borders of 

 the basin. These folds have their steepest slopes toward the margin, the 

 sides toward the center of the basin being much less inclined, so that 

 by erosion of the declivities of the anticlines on either side a relatively 

 broad trough is formed in the central part of this field. So far this relation 

 of the slopes of the upper folds to the margins of ancient massive rocks has 

 been distinctly traced only in the northern half of the mountain-built 

 district, but there are indications that it exists also in the southern portions 

 of the field, being there concealed by the waters of the bay or masked by 

 the prevailing covering of drift. 



The disposition of the strata as above noted appears to require the 

 supposition that the thrust acted from either side in such a manner that the 

 central portion of the basin was a relatively neutral zone in the vaulting 



