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 
ereat 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 
