OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 455 



that it represents the stratified rocks leaning against the walls of the great granitic 

 central dykes, at steeper and steeper angles, the higher we ascend towards the 

 summits. It is inexact, however, in picturing the granitic nucleus of the anti- 

 clinal mountain, as a wedge or broad prism tapering upward, for reasons already 

 shown. Undoubtedly such a mountain, if we can imagine it denuded or truncated 

 to lower and lower levels, would disclose a progressively increasing quantity of 

 intrusive igneous rock, but this would be in the multiplication of the lateral 

 granitic injections, and it is only in this erroneous sense, that the igneous nucleus 

 can be regarded as a prism. Its cross section is branching rather than wedge- 

 like. 



The Upward Movement of an Igneous Dyke would tend to Stretch and not to Corrugate the 



Flexible Strata. 



The view here admitted of the elevation of the igneous nucleus of a mountain, 

 along with the strata which mantle it, while it is perfectly compatible with the 

 hypothesis, to be hereafter advanced, of the origin of anticlinals generally, is wholly 

 inconsistent with the somewhat current notion of the mode of origin of undulations 

 and plications in the stratified rocks, by pressure from the tangential horizontal 

 thrust of such uprising igneous axes ; so far from its producing a lateral corrugating 

 pressure upon the strata adjoining, and resting against it, a central granitic or 

 other igneous dyke lifted vertically by one or many successive movements, pa- 

 roxysmal or gradual, would rather stretch or distend the strata as it carried them 

 upward than compress them. 



Theory of Upward Tension against Lines or Points of the Crust. 



Another common theory of crust movement and elevation of anticlinal belts 

 supposes, vaguely, an upward tension or stretching of the crust of the earth along 

 one or several lines, or at one or several focal points, without attempting to ac- 

 count for the linear or focal force, or to assign a cause for the restricted limits 

 within which it is assumed to act. This conception, though confessedly indis- 

 tinct, is frequently appealed to in explanation of the lifting of mountains, the 

 corrugation of strata, and even the formation of regular groups of parallel anti- 

 clinal waves. I propose to consider its weak points. 



Any theory henceforth admissible into physical geology, must explain the now 

 clearly established general fact of the regular wave structure of the earth's dis- 

 turbed zones. But this wave structure cannot be interpreted on the mere sup- 

 position of simply an upward pressure exerted either along one or many lines. The 

 peculiar configuration of the crust waves, shown in this paper to be characteristic 

 of them in all undulated regions, requires an hypothesis which will furnish both 

 an undulating and a horizontal tangential motion ; moreover, the ordinary doc- 

 trine, if it assumes the pressure from beneath to be exerted along a single line at 



VOL. XXI. PART III. 6 G 



