238 RECOKD OF SCIENCE FOR 1887 AND 1888. 



tions have corrected certain early inferences as to the mechanism of such 

 faulting; inferences originating partly in Europe and partly in the Ap- 

 l)alachians, and current for many years on both sides of the Atlantic. 

 Heim noticed that in the Alps the inverted limb of an anticlinal is 

 stretched or even crushed out between the anticlinal and synclinal 

 curves of an overturn and the flexure thus converted into a fracture, 

 and inferred that overthrust faults are always formed in this manner; 

 but Willis points out that this explanation fails to account for many 

 of the faults of the Apjyalachians because the essential fact of squeezed 

 beds has not been found in that region. He shows also that the Appala- 

 chian sedimentary series, from the Cambrian ujjward, is composed of 

 strata differing greatly in their capacity for resistance to horizontal 

 thrusts, and that these variations in rigidity occur from place to i)lace in 

 the same strata as well as in the different strata superimposed one on the 

 other, and that the rigid strata may not fold at the place where a verti- 

 cally adjacent flexible stratum does fold, but that the rigid stratum may 

 ride forward on its bedding plane until it reaches an axis (anticlinal or 

 sync^linal) in which both beds have suffered flexure, and that the forward 

 movement may then sheer across the beds on the opposite dip, produc- 

 ing a fault.* Under this view it would appear that the yielding of 

 rocks to horizontal pressure may take place, (1) as corrugation ; (2) as 

 overthrusts, perhaps originating in incipient corrugations ; (3) as vari- 

 ous combinations of corrugation and overthrust faulting, the difference 

 in effect depending upon difference in structure, difference in the pressure 

 beneath superincumbent beds, and other differences in conditions. 



Willis's inferences from A])palachian structure have been checked by 

 exi)erimentation. During the past year he has subjected masses of wax 

 consisting of alternating layers of varying rigidity, built up in imitation 

 of the rocky strata of the earth's crust, to horizontal compression, the 

 waxen strata being variously loaded in different experiments ; and he 

 finds that the deformation of the miniature strata in his models imitates 

 the deformation displayed on a grander scale in the Appalachian Mount- 

 ains. 



Another order of faults, also resulting from horizontal compression, 

 has recently been developed by Davis. There are in the Connecticut 

 Valley extensive deposits of sandstone and shale of Triassic age, of un- 

 known thickness, generally dipping eastward at a considerable angle ; 

 and there are in New Jersey, Pennsylvania, and northern Virginia sim- 

 ilar deposits of Triassic sandstone, also of unknown thickness, dipping 

 westward at high angles. In both of these areas the exi;steuce of faults 

 has long been suspected, and in a few cases minor faults have actually 

 been discovered ; so the feeling has gained ground that despite the persis- 

 tent and high dips over broad areas, the deposits are onlycf limited thick- 

 ness. !Now Davis has shown t that in the Connecticut area there are 



*Bull. Phil. Soc. Wash., 1889, vol. xi (in press). 



1 7th Anu. Rpt. U. S. Gool. Survey, 1888, pp. 461-490 



