252 F. H. Lahee — Metamorjphism and Geological Structure. 



between such factors as mineral composition and intensity of 

 deformation, or degree of deformation and stratigraphic depth, 

 and the like, obviously cannot be measured with precision. 

 Yet a broad, general quantitative dependence can be deter- 

 mined. 



Structural Geology of the Carboniferous Formation. 



Theoretical Considerations. — Before describing the struc- 

 tural geology of the basin, let us see what variations in inten- 

 sity of folding may be expected, on theoretical grounds, in a 

 region of deformed strata. 



A fold, in the geological sense, is the expression of variable 



Fig. 2. 



Fig. 



plane. 



2. Diagram of an elliptical quaquaversal anticline, a-e-b-f, axial 



a-b, direction of operation of minimum component of force. 

 c-cl, direction of operation of maximum component of force. 



compressural forces which have operated upon variable resist- 

 ances. In most cases the pressure has been applied along 

 approximately horizontal lines. However numerous the forces 

 may have been, they may be regarded as having been equiva- 

 lent to two components, — one of maximum value, which acted 

 parallel to the greatest compression, and one of minimum 

 value, which acted parallel to the least compression, at right 

 angles to the first. 



The simplest illustration is the quaquaversal anticline (see 

 fig. 2). Here a-b is the direction of the minimum component, 

 and c-d is that of the maximum component. Pitch, measured 

 along the slopes e-a and e-b, is really dip in the axial plane, 

 a-e-b-f. Whenever there is a pitch — and it may be stated as a 



