380 



FORMATION OF MOUNTAINS. 



a series of experiments on the effects of lateral 

 pressure, in which the conditions of nature 

 were better preserved than in the famous ex- 

 periments of Sir James Hall with various cloths 

 folded and held down by the superposition 

 of heavy weights, which he made to assume 

 shapes analogous to the disposition of masses 

 of rocks by squeezing them from each side. 

 M. Favre used a homogeneous and plastic ma- 

 terial, and left the upper surface free to as- 

 same the forms which would result from lat- 

 eral contraction, and which resembled hills 

 and mountains. He took sheets of India-rub- 

 ber, 16 millimetres in thickness and 40 by 12 

 centimetres in area. Stretching them to a 

 length of 60 centimetres, he covered them with 

 a layer of soft pottsrs' clay, which varied in 

 thickness in the different experiments from 25 

 to 60 millimetres. The contraction of one 

 third which he produced in most of his ex- 

 periments corresponds to that which has ac- 

 tually taken place in certain parts of Savoy. 

 The strata in the mountains between Dessy 

 and the Ool du Grand Barnaud, for instance, 

 are folded and contorted into two thirds the 

 compass which they had before compression 

 when lying horizontal. If the solid crust of 

 the earth rests upon a pasty nucleus whose 

 volume is slowly and constantly diminishing, 

 then the hard, rocky mass will be broken and 

 thrust out of shape as the interior supporting 

 mass contracts. 

 Such a dislocation of the external strata has 



taken place at different geological periods and 

 in all parts of the world. In many places the 

 lower strata are distorted, while the more re- 

 cent overlying formations are unbroken and 

 horizontal. Often the contraction has been 

 much greater than that obtained in the experi- 

 ments. In some places the folds are very close 

 together and their sides nearly vertical. These 

 disturbances must have occurred with imper- 

 ceptible slowness, as they follow the contrac- 

 tion by minute degrees of the earth's radius. 



Drawing horizontal lines on the side faces 

 of the clay to indicate the position which strata 

 would take after compression, and confining 

 the extremities of the clay band within verti- 

 cal wooden strips, he allowed the caoutchouc 

 to contract, thus producing lateral compres- 

 sion by the double means of the adhesion of 

 the clay to the caoutchouc and the pressure ot 

 the wood supports. The contraction of the 

 caoutchouc alone would produce only slight 

 wrinkles on the surface of the clay, and the 

 action of the wooden ends alone would only 

 cause the clay to bulge at the extremities, pro- 

 ducing no effect in the center. The pressure 

 secured in Professor Favre's experiments pro- 

 duced contortions of strata very similar in con- 

 figuration to the earth's surface. In some 

 places the plane surface of the clay was ele- 

 vated without being broken or thrown much 

 out of the horizontal position ; in others vaults 

 and plications were produced which resemble 

 hills, mountains, and valleys. The ridges some- 



Fio 1. 



times approached the perpendicular ; at times 

 they were complete folds, at other times they 

 separated at the summit in a longitudinal frac- 

 ture increasing in width downward, and at the 

 base in a complementary fracture widest at 

 the mouth. The strata are broken in places, 

 producing fissures and caverns below the sur- 

 face, or are crossed by clefts or faults, some- 

 times vertical, sometimes inclined. The con- 

 tortions do not extend perpendicularly across 

 the clay band, but take various directions, the 

 opposite sides showing different deformations. 



The experiment shown in Fig. 1 was with a 

 band of clay about 25 millimetres thick, which 

 after compression attained a thickness at the 

 culminating point of 62 millimetres. It repro- 

 duces many of the common features of the 

 Jura, the Alps, and the Appalachians. At a 

 is a vault slightly broken at the summit, cov- 

 ering a cavern ; J is a valley open at one end 

 and closed at the other; e is a vault, nearly 

 straight and of even elevation ; g and h are 

 twisted and slightly broken vaults ; and i is a 

 ruptured fold whose sides are nearly vertical. 



