MECHANICS OF FORMA TION OF A RCUA TE MOUNTAINS 1 69 



in sufficient intensity to produce appreciable deformation or 

 folding may be referred to as the reach. In rock masses which 

 offer a uniform resistance to folding throughout a given area, the 

 limited reach determines that folds will begin to form upon the side 

 which is toward the active force. ^ The very existence of an area 

 which folds, surrounded by one which does not, imphes that but for 

 this central folding area the rock masses would be competent to 

 transmit the tangential force without extensive deformation. The 

 fact that folds first develop upon that side of the folding area which 

 is toward the active force is amply demonstrated by simple experi- 

 ments which were performed by Daubree.^ In these experiments 

 a vertical section of the unyielding and encompassing area of the 

 earth's shell was represented by a stiff piston-rod through which the 

 active force was transmitted to a flexible leaden strip which 

 therefore takes the place of the folding rock masses. When the 

 strip of lead was made of uniform thickness, and hence of uniform 

 strength, the folds within it formed first upon the side which was , 

 toward the piston. Only by thinning and thereby weakening the 

 strip at the farther end could the folds be first produced at that end. 

 The experiments of all later investigators working with materials 

 and under conditions which must more nearly simulate those 

 obtaining within the earth's shell have only confirmed the correct- 

 ness of these simple deductions from Daubree's experiments. 



It is because of this limited reach of the deforming stress that 

 antichnoria which by construction imply the simultaneous develop- 

 ment of similar and approximately equal anticlines throughout the 

 length of a flatly extended arch of strata^ are apparently unrealized 

 in nature. Most of the supposed classical examples have been 

 drawn from the Alps as represented upon old sections, and these 

 may today be adequately explained upon the assumption of the 

 development of successive anticlines as detailed below (p. 172). 



The strength of rock formations as modified by temperature and 

 load no doubt sets a definite limit upon the initial span of anticlines 



' See note on Paulcke's experiments on p. 172. 

 ^ A. Daubree, Geologie experimentale, pp. 292-300. 



3 See, for example, Van Hise, Principles of North American pre-Camhrian Geology, 

 pp. 608-9. 



