440 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 
the curves become, in many instances, broad, depressed, and al- 
most symmetrical in form. 
From the descriptions here given of the structure of the 
Appalachian chain and other disturbed districts, it is obviously 
a general law, that the axis planes of the flexures are not only 
inclined all in one prevailing direction, though at different angles, 
but that they dip invariably towards the quarter or zone of maxi- 
mum disturbance and rupture of the crust. 
FRACTURES OR Fauuts IN TRACTS OF UNDULATED STRATA. 
Two classes of dislocations abound in all zones of plicated and 
undulated strata, where the crust waves exhibit much steepness, 
and especially where they have the inverted or folded form. By 
far the most numerous, though the shortest and least conspicuous 
class, are the breaks or faults which run approximately trans- 
verse to the strike of the anticlinal and synclinal axes. These 
may be extensively recognized in the Appalachians, where they 
are a primary cause of the deep ravines, or breaches through the 
ridges, which furnish passage to nearly all the rivers, and even 
lesser streams which drain this chain. Such ravines are especially 
frequent near the extremities of the large anticlinal waves, par- 
ticularly where they have been cut through along their crests 
by denuding waters, and have given rise to valleys of elevation 
and erosion, inclosed by monoclinal, outward-dipping, sandstone 
ridges. It would seem as if the elliptical folding round of the 
strata towards the ends of the great denuded waves had caused 
the horizontal wrenching which resulted in these fractures. Mr 
Wititam Hopkins, of Cambridge, has, in an able paper on the 
subject of dislocations affecting dome-shaped elevations of the 
earth’s crust, indicated the true source, I conceive, of the double 
system of fractures to be met with in all elliptical anticlinal belts. 
An elongated anticlinal wave is, in truth, only a greatly lengthened 
elliptical dome, in which the radial cracks caused by a maximum 
tension in the strata transmitted from the more central portion 
of the crust-wave, are distributed, some of them longitudinally, 
others transversely, as respects the anticlinal axis, the transverse 
ones multiplying themselves where the elliptical strain has been 
greatest, towards the two extremities of the waves. 
The other far more conspicuous class of dislocations connected 
with these crust undulations, are the great longitudinal ones. 
These are of frequent occurrence in the more contorted portions of the Appalal 
Fig. 1. 
Generalized Section of the Appalachian Chain from north-west to south-east, through the Juniata district of Pennsylvania. 
