OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 463 



the anticlinal axes of his cleavage curves. Now, just the reverse of this steepen- 

 ing of the cleavage planes towards the regions of chief metamorphism, will be 

 found to be the real law of gradation in the Appalachians, the Alps, and the district 

 of the Ardennes and Southern Belgium. Obedient to a law already explained, the 

 cleavage dip, following the dip of the axis planes of the flexures, is not most but 

 least inclined in the districts most convulsed, and grows progressively steeper, 

 as we advance across the undulations to the districts of minimum disturbance. 

 In the Alps the plications lie flattest next the high central crests of the chain, 

 and there the cleavage dip is often at a very low angle ; but receding towards 

 the plain of Switzerland, where the theoretical view requires that it should 

 be flatter, it is really steeper, and even approaches to perpendicularity; and 

 precisely analogous is the gradation when we cross the Appalachians from 

 south-east to north-west. Generalizing the dips of the cleavage planes on both 

 sides of a double belt of flexures like that of the Alps, and excluding the central 

 crests, where the jointage of the igneous rocks, and the cleavage structure im- 

 pressed by them is more vertical, the real curve of dip for the whole zone will be 

 found to be a synclinal one, and not the two halves of two anticlinals, the gener- 

 ating axes of which are far outside the chain, one in the plain of Switzerland, 

 the other in the plain of Northern Italy. 



I am much gratified to find, that my objections to the mechanical theory of 

 cleavage find support in the able writings of Professor Sedgwick, who, in a note 

 in his " Synopsis/' states several cogent reasons for rejecting the hypothesis. 

 While some of my own objections are but an expansion of those presented by this 

 eminent geologist ; others are independent of his, growing out of my own observa- 

 tions. This accordance gives me additional confidence in the soundness of the 

 generalizations upon which they rest. 



Theoretical Views. 

 Theory of the Flexure and Elevation of Undulated Strata. 



The wave-like structure of the Appalachians and other undulated zones, has 

 been attributed by the author and his brother, W. B. Rogers, in their communi- 

 cations to the American Association in 1842, and to the British Association in the 

 same year, to an actual undulation of the supposed flexible crust of the earth, 

 exerted in parallel lines, and propagated in the manner of a horizontal pulsation 

 from the liquid interior of the globe. We suppose the strata of such a region to 

 have been subjected to excessive upward tension, arising from the expansion of 

 molten matter and gaseous vapours, the tension relieved by linear fissures, through 

 which much elastic vapour escaped, the sudden release of pressure adjacent to the 

 lines of fracture, producing violent pulsations on the surface of the liquid below. 

 This oscillating movement in the fluid mass below would communicate a series of 



VOL. XXI. PART III. Q I 



