THE FORMATION OF MOUNTAINS, 463 



the influence of the caoutchouc alone, without the presence of the 

 supports, there are formed only slight wrinkles on the surface of a 

 sheet of clay 3 or 4 cm. in thickness ; and if the supports alone com- 

 pressed the clay placed on a material which is not compressed (a very 

 smooth oiled plate), the clay scarcely wrinkles near the center of its 

 surface ; it increases a little in thickness and forms swellings (boiir- 

 relets) against the supports. The strata which appear to divide the 

 masses of clay, and whicli are represented in the figures, are not really 

 strata, but simply horizontal lines at the surface of the clay." 



Such pressure as has been applied in these experiments produces 

 contortions of strata which elevate the surface of the matter com- 

 pressed, as well in the plane parts or plains, as in those which take the 

 forms of valleys, hills, or mountains. These latter have the appearance 

 of vaults or folds, sometimes perpendicular, sometimes warped [d^- 

 Jetes) ; the ridges are complete, or broken at the summit by a longi- 

 tudinal fracture, narrow below and wide above ; next, another frac- 

 ture, narrow above and wide below, is produced at the base of the 

 mountain or vault. The sides of valleys are sometimes almost vertical, 

 sometimes present gentle slopes. The strata are less strongly con- 

 torted in the lower parts than in the neighborhood of the upper sur- 

 face. They are disjoined in certain parts by fissures or caverns ; they 

 are traversed by clefts or faults inclined or vertical. All these defor- 



FiG. 4. 



mations are the more varied in that they are not similar on the oppo- 

 site sides of the same band of clay. 



Most of these phenomena are seen in Fig. 1, which represents the 

 result of an experiment made on a band of clay whose thickness before 

 compression was about 25 mm., while after that it attained 62 mm. at the 

 culminating point. At a is seen a vault a little broken at the summit, 

 covering a cavern similar to that figured in the memoir of Sir J. Hall 



