434 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 



sylvania, and in Northern Vermont, it will be found to arise from the interference 

 or interlocking of the ends of the waves of different but adjacent segments. 



3. Crossing any great belt of anticlinal and synclinal flexures, such as that of 

 the Appalachians, or that of Belgium and the Rhenish Provinces, it will be no- 

 ticed, when the undulations are carefully traced and compared, that these consist 

 of more than one class as respects dimensions ; indeed they will be found to be 

 of two or three grades, when grouped according to their length, height, and am- 

 plitude. In most parts of the Appalachian chain, there are at least two prevail- 

 ing magnitudes in the waves. The chief class, or primary undulations, are of 

 great size, their length amounting to from 50 to 120 miles, and their breadth to 

 several miles, except where they are closely compressed. The subordinate or 

 secondary waves are seldom more than a fourth of a mile wide, nor do they 

 usually exceed ten miles in length, and in many groups they are much shorter. 

 Frequently a third class is to be met with, of still smaller and less persistent 

 flexures, — rolls of the strata, as they are called in the coal-mining districts of 

 Pennsylvania, — which seem to be only local corrugations of the more superficial 

 rocks, and not true undulations of the crust pervading the entire thickness of 

 the formations. The relations of the primary to the secondary waves will be 

 enlarged upon hereafter. It will suffice, under the present head of parallelism of 

 flexures, to state that, for the Appalachians at least, those of the second order 

 are not necessarily parallel to those of the first, though within a given district 

 they observe among themselves the same mutual parallelism which the larger 

 or primary waves exhibit. 



Forms of the Waves. 

 Symmetrical Flexures. 

 The individual waves or flexures of a belt of undulated strata occur under 

 three essential varieties of form. The first, or most simple, is that of a convex 

 or concave wave, or in technical geological language, an anticlinal or synclinal 

 flexure, in which the two slopes of the wave are equal in their degree of incur- 

 vation or steepness. This symmetrical form is restricted chiefly to the gentler 

 or flatter undulations, and especially to those of considerable amplitude. We do 

 occasionally meet with steep waves of the strata, having a nearly equal inclina- 

 tion on both their sides ; but these are generally broken curves, exhibiting a snap 

 or sudden angle at the anticlinal or synclinal axis, in place of the gradual arch- 

 ing, which is the normal form of all regular crust undulations. 



Normal Flexures. 



Another and more prevailing form displays a more rapid incurvation, or steep- 

 ening of the flexure, on one side than on the other. Waves of this type have 

 been called Normal Flexures by my brother and myself, in our descriptions 



