194 APPENDIX. 



we pass to that of a great mountain chain, the want of some guiding principle in the inter- 

 pretation of complicated contortions becomes greatly felt. It is to men such as De Beaumont, 

 Hopkins, and Haughton that we must look for the fit discussion of this subject. Mr. Hopkins 

 gives very little encouragement to the attempt. In order to study the comparison of actual 

 with theoretical results, he tells us that he chose the area of the Weald " on account of the 

 regularity of its boundary and the apparent absence of the effects of that more violent, 

 local, or irregular action of elevating force, which it must ever be impossible to reduce to 

 calculation" (p. 1). 



Professor H. D. Kogers (" Geology of Pennsylvania," 1858) has paid more attention to 

 detail of stratigraphy than did either De Beaumont or Hopkins. 

 H.r>. Rogers' theory of dis- jj; s pi T ,j ons are principally based upon the study of the Appala- 

 chian mountains of America ; but, both from observation and 

 from written description, he attempts to affiliate other regions to the same laws of structure. 

 The Appalachians are described as one chain, demonstrably elevated at one epoch. The 

 ranee is 1 ,500 miles long by 150 broad, but is divisible longitudinally into eleven sections, 

 five of which have a curved axis, and six are straight ; three of the latter have an approxi- 

 mate north-south direction, and three run nearly east-west or almost at right angles to the 

 others. Mr. Rogers points to these facts as incompatible with M. de Beaumont's theory of paral- 

 lelism and synchronism. Throughout the Appalachians the strata are waved, the undulations 

 observing a parallelism among themselves, and with the igneous axis of the district in which 

 they occur. In most parts of the chain waves of two magnitudes are to be found; the larger 

 of these attain great dimensions, being from 50 to 120 miles long, by several miles 

 wide ; the subordinate or secondary waves are seldom more than ten miles long, by 

 half a mile wide. These latter are regarded as only local corrugations of the superficial rocks, 

 not true undulations of the crust. Parallelism does not necessarily obtain between these 

 systems. 



In tli&from of waves Mr. Rogers distinguishes three essential types. The first is symmetri- 

 cal flexure, a curve of which both slopes are equal. This variety is generally restricted to 

 the gentler undulations. The second kind is the normal flexure . It displays a greater steep- 

 ness on one side than on the other. This form prevails where the forces of disturbance were 

 neither intense, nor yet feeble ; and it is found to occupy an intermediate position geogra- 

 phically between the two other varieties. As a general rule, in any one region the steeper 

 slopes are all in the same direction. The normal flexure attains its limits when the steeper. 

 side becomes vertical. Beyond this limit we have the third class, called folded flexure, in 

 which the strata on one side of the axis of the curve have been displaced beyond a vertical 

 position, so as to approach in parallelism to the strata on the other side of the axis. As 

 in the case of normal flexures the more incurved sides of the folded waves in any district 

 generally slope in the same direction. 



Regarded longitudinally, each wave has its maximum height in the middle of its length. 

 The form also may change : from being of the folded variety in its middle portion a wave 

 may become normal and symmetrical towards its terminations. Starting, in any group of 

 flexures, from the side of maximum disturbance, which is also invariably the quarter of 

 greatest igneous action, the waves exhibit a similar gradation, and at the same time an 

 expansion in the width of the wave. The generalised section of the Appalachians 



