500 OF THE PHYSICAL STRUCTURE 



within the notice of geologists. Before a philosophical theory of 

 flexures can be framed, large opportunities must be had for clas- 

 sifying their phenomena, and tracing their laws of gradation. 



It is a curious and important fact, connected with this group of 

 axes, that in certain cases, chiefly, we believe, in wide and deep 

 troughs, the included smaller axes or wrinkles, though parallel to 

 each other, are not parallel to the general synclinal axis of the 

 basin, in which they occm*. This feature is obvious in all the 

 deep anthracite coal-basins of Pennsylvania, especially near their 

 terminations. These lesser, subordinate axes, generally have a 

 strike parallel to that of one of the great flexures bounding the 

 basin ; but, on account of the convergence of the si^es of the 

 ti'ough, they are necessarily more or less oblique to the opposite 

 margin. They are, therefore, so many long, parallel warpings of 

 the strata, conforming to one boundary, but abutting acutely 

 against the other. Sometimes, indeed, they cross the basin very 

 gradually, or pass almost longitudinally, from one side to the 

 opposite, and die out, as wrinkles on the slopes which bound the 

 basin. That they have originated in an inequality in the energy of 

 the linear forces concerned in bending and elevating the rocks 

 along the principal flexures, and arise, therefore, from an actual 

 warping of the strata, seems altogether probable. If so, they are 

 secondary consequences of those more general and extended 

 movements, which give existence to the grander flexures, in whose 

 folds they lie. To describe all the phenomena relating to these 

 minor assemblages of axes, the full investigation of which, as it 

 concerns the mining operations of our coal-fields, is, perhaps, the 

 most useful practical inquiry that the geologist can undertake, 

 would demand a body of minute details, only to be elucidated by 

 a general map of the flexures, not yet ready for publication. 



5th. Parallelism of the axes hi each group. The parallelism of 

 the several axes or lines of flexure, which compose a group, either 

 extensive or limited, is one of the most remarkable relations. 

 The descriptions already furnished show, that it prevails in every 

 portion of the chain, whether straight or curved, and extends even 

 to the members of the smallest groups. A striking exhibition of 

 this mutual parallelism may be noticed among the inverted and 



