436 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 
or of incurvation of the two slopes of the wave, modified, according to a certain 
law of variation, with the dip. In the third class, or that of flexures with inver- 
sion, the axis planes are likewise not perpendicular ; and it will be found that, 
in the great majority of instances, they dip with a less degree of steepness than 
the planes bisecting waves of the normal or other unsymmetrical type. Indeed, it 
may be stated generally, that, just in proportion as the flexure departs from the 
symmetrical wave form, through greater and greater inequality of dip, up to paral- 
lelism of the inverted with the uninverted branch of the curve, the axis plane 
departs from the perpendicular direction, to assume a less and less inclination to 
the horizon. In many districts of extreme plication of the strata, for instance in 
the Atlantic slope of the middle and southern States of North America, also in 
the Bernese Oberland, in the Ardennes, and in North Wales, the axis planes dip 
at an extremely low angle, consequent on the excessive amount of horizontal 
movement which the strata have undergone in the act of folding. 
So nearly parallel are the inverted to the uninverted sides of the folds,—the 
axis planes all, of course, dipping one way,—in many districts of close plication, 
that the detection of the anticlinal and synclinal bends is not a little difficult, 
especially where the sections, natural or artificial, are not perfectly clear of 
superficial debris. In such cases the whole plicated mass looks as if it contained 
but one dip, or consisted of only one thick sequence of deposits, instead of am uch 
thinner formation many times reduplicated. To add to the liability of error, such 
bodies of folded strata are especially subject to that condition of jointage which is 
called slaty cleavage. In this structure, as I shall presently show, the divisional 
planes not only tend to obscure the original planes of sedimentation by their 
ereater conspicuousness, but they often, by observing a very prevailing parallel- 
ism to the general dip of the folded beds, or, more strictly, to their axis planes, 
effectually disguise the anticlinal and synclinal curves. It is from these circum- 
stances, and not from any erroneously supposed effect of truncation or denuda- 
tion, actually to remove the anticlinal bends of the strata, that it is frequently 
so difficult to detect the true order of original superposition and the real thick- 
ness of closely plicated formations. Of course, no erosion upon an anticlinal 
axis, however closely folded it may be, can obliterate the bends in those beds 
which have their curves below the level reached by the denuding agent. 
Crust Waves Straight and Curvilinear. 
Regarding the great flexures of the crust as individual waves, which in truth 
they seem to be, we find them exhibiting, not only the above differences in the 
sloping of their two sides, but marked differences of form when viewed longitu- 
dinally. Thus, many are of extraordinary straightness; some of the larger simple 
anticlinals of the Appalachians being more than 100 miles in length, without any 
material or even perceptible horizontal crooking or deviation in their crest lines. 
