MECHA NICS OF FORMA TION OF A RC UA TE MO UNTA INS 1 7 1 



friction on bedding planes, the viscosity of the strata, and any opposing force, 

 as that of the load; the latter becomes active when it can cause some part of 



the resisting mass to move 



In strata under load an anticline arises along a line of initial dip, when a 

 thrust, sufficiently powerful to raise the load, is transmitted by a competent 

 stratum. The resulting anticline supports the load as an arch, and being 

 adequate to that duty it may be called a competent structure. From the 

 conditions of the case it follows that none other than a competent structure 

 can develop by bending. If the thrust be not powerful enough to raise the 

 load there will be no uplift ' 



What has been brought out above concerning the direction of 

 the active force would indicate that, coming as it does from the 

 ocean basins, the thrust, instead of being influenced by initial dip 

 at the shoreward end of a section of epicontinental deposits (which 

 should develop there an initial syncline) , is received at the off-shore 

 end of the section and should be first diverted upward by the 

 steeper beds upon the continental slope and so yield an initial 

 anticline. This is, moreover, more in harmony with the results 

 of experiment. 



It seems proper to speak of the competence of an arch or anti- 

 cline as its capacity in any stage of formation to lift load resting 

 upon it.* 



In order to simulate rock folding under as nearly as possible 

 natural conditions, Willis in his experiments made use of layers 

 which were of nearly uniform thickness and strength throughout 

 and which, though sufficiently rigid to be deformed by failure under 

 ordinary testing conditions, became potentially plastic under the 

 load of shot which was applied in the experiments. In confirmation 

 of the results at which one arrives from a purely theoretical treat- 

 ment of the subject, it is important to note that in all Willis's 

 experiments except when special conditions were introduced, 

 anticlines developed on the side of the mass toward the active force 



' Tlte Mechanics of Appalachian Structure, pp. 246, 250. 



^ It is little likely that, barring exceptional cases of small anticlines, a competent 

 stratum can lift the entire load from beds beneath, as seems to have been implied by 

 WilHs. We should in that case approach to surface conditions within a well-cemented 

 masonry arch in which the accelerated rate of increase of weight of arch relative to its 

 strength sets such a low limit upon the span as to be prohibitive. 



