VERTICAL COMPONENT OF DEFORMING FORCE. 233 



Strain in the lower layers; and how the intensity of the flexing diminishes 

 away from this line northward to the Boston Mountains, as illustrated by 

 figure 1. 



But, in addition to presenting evidence of lateral movement, the warped 

 character of the structure of the Arkansas valley, described on page 230, 

 suggests the action of other causes. To produce such a warped surface, a 

 considerable vertical and upward component in the force acting is neces- 

 sary. The shells of successive strata constituting these quaquaversal arches 

 must have been stretched or elongated. Gilbert has explained how this was 

 produced by laccolite intrusion in the formation of the Henry mountains.* 

 Here we have no reason to suspect the proximity of such masses of igneous 

 rock. Conditions can be conceived by which lateral compression from two 

 directions might alone produce this; but equally well, if not better, can we 

 attribute this warping to expansion of the lower strata through an increase 

 of temperature due to a rise of the isogeotherms with continuous sedimenta- 

 tion, as explained by Reade. A condition of tension in the upper layers 

 would thus be produced ; the topmost of these layers would be fractured and 

 pulled asunder ; the lower lying ones would be compressed and caused to 

 flow and spread under the influence of great pressure. The amount of frac- 

 turing and the amount of spreading in any one case would be dependent 

 upon the elasticity and the plasticity of the layer and upon the amount of 

 pressure it is subjected to. The plications would thus represent the com- 

 bined effects of two causes : Firstly, to the action of the general cause of 

 expansion of the lower beds is due the production of the warped surface ; 

 secondly, to the superior activity of this first cause in the region to the south 

 is due a lateral movement towards the north which, acting as a secondary 

 cause, produced unsyrametrical folds and lateral compression of the rocks. 



In connection with this question of the origin of quaquaversals, a sugges- 

 tion is in place relating to the value of developed cross-sections constructed 

 from observed dips. From such cross-sections the amount of linear com- 

 pression in the plications of the Alps has been estimated at 72 miles, and in 

 the case of the Appalachians at 88 miles. These figures have been criticised 

 by geologists and thought to be excessive — more than the supposed causes 

 were adequate to produce. 



On the assumptions that the rocks are in a state of strain ; that the upper 

 layers of an arch are in a condition of tension ; and tjiat the bulging is pro- 

 duced by an effort of the compressed lower layers to protrude themselves, 

 the top layers would undoubtedly be fractured. The forces of degradation 

 would attack the arch vigorously along the lines of such fractures, would 

 gradually remove the dome-like cover, and would then proceed to cut fur- 

 ther down into the core. Such cutting means, however, removal of material, 



*Geology of the Henry Mountains, 1877, p. 76. 

 XXXV— Bull. Geol. Soc. Am., Vol. 2, 1890. 



