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



we find the north-west sides of the waves, with few exceptions, steeper than the 

 south-east ones, not only where they are inverted, but where they have a nor- 

 mal dip. We find, moreover, as we advance, that the waves grow more and more 

 open, and that the distances between them increase, that they subside in height, 

 and that the two slopes approximate nearer to equality. These gradations are 

 admirably disclosed in any traverse across the strike north-westward, from the 

 water-shed of the Ardennes to the Belgian coal-fields of the Meuse. I can detect, 

 in the features of Dumont's exquisite map of Belgium and the neighbouring 

 countries, the very same relations of the longitudinal faults to the flexures which 

 have engendered them, as those above described in the fractures of the Appala- 

 chians. They evidently occur, for the most part, on the north-west sides of the 

 anticlinal axes, and cause older strata to ride upon newer ones plunging under them, 

 with approximately parallel dip. Even the phenomena of cleavage, presently to 

 be described, will be seen to exhibit the very same laws in the more metamorpbic 

 southern half of this wide zone of plication, which they present along the south- 

 eastern side of the Appalachian chain, and the Atlantic slope bordering it. This 

 region of the Rhenish Provinces and Belgium further agrees with the Appalachians, 

 in being a zone of undulations and plications, where the folding movement has 

 been all in one direction. 



The Jura Chain of Switzerland. — The Jura chain of Switzerland, as I pointed 

 out in 1848, in communications to the Geological Society of London, and in 1849 

 to the American Scientific Association, is another very interesting belt of crust 

 waves, displaying, in its structure, a close resemblance to the Appalachians. 



It embraces, like the American mountains, many groups of waves differing in 

 the directions of their axes in different districts of the chain, but the individual 

 groups composed of waves which are remarkably parallel. Few of these undu- 

 lations exhibit actual inversion of their steeper sides, the dip only in some in- 

 stances passing the perpendicular, and generally not exceeding on an average 70°, 

 the gentler or opposite slopes having a mean slant of about 40°. In four traverses 

 which I made across this chain, I observed one almost invariable law as to the 

 direction of the steep and gentle sides of the undulations, or, in other words, of 

 the axis planes. Contrary to first anticipation, and to the belief of many Swiss 

 geologists, I found the steeper curvature of the waves directed toward the Alps, 

 and not from them, implying that the crust movement which lifted these grand 

 and picturesque arches proceeded from the north-west, and not from the chain 

 of the Alps. This also is a belt whose undulations are chiefly in one direction. 



The Alps. — The great chain of the Alps is much more complex in its struc- 

 ture than either of the undulated zones yet described. It contains but few waves 

 of the open or normal type, but innumerable close foldings or plications. Through- 



