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 
im 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 
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. 
| pressure upon the strata adjoining, and resting against it, a central granitic or 
| 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. 6G 
