Dec. 5, 187S] 



NATURE 



105 



the vertical, and are close against each other ; this struc- 

 ture indicates that pressure has been exercised in a 

 stronger manner than I have indicated. 



" These powerful lateral thrusts of the external and solid 

 parts of the globe appear to result from a diminution 

 which the radius of the interior pasty or fluid nucleus has 

 undergone during millions of ages. It may have been 

 sufficiently great to cause the solid crust (which must 

 always have been supported on the interior nucleus, whose 



volume continually diminishes) to assume the forms which 

 we know, with a slo^vness equal to that of the contraction 

 of the radius. 



" To return to my experiments. At the extremities of 

 the band of clay are pieces of wood or supports, which 

 accompany it in its movement of contraction. The clay 

 is thus compressed at once by its adhesion to the caout- 

 chouc and by lateral pressure of the supports. By the 

 influence of the caoutchouc alone, without the presence 



Fig. s* 



of the supports, there are formed only slight wrinkles on 

 the surface of a sheet of clay 3 or 4 ctm. in thick- 

 ness ; and if the supports alone compressed the clay 

 placed on a material which is not compressed (a very 

 smooth oiled plate), the clay scarcely wrinkles near the 

 centre of its surface ; it increases a little in thickness 

 and forms swellings {bourrelets) against the supports. 

 The strata which appear to divide the masses of clay, 

 and which 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 experir 

 ments produces contortions of strata which elevate the 

 surface ,of the matter compressed, 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 {dejetes); the ridges are complete* 



Fig. 



or broken at the summit by a longitudinal fracture, 

 narrow below and wide above ; next, another fracture, 

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

 the mountain or vault. The sides of valleys are some- 

 times almost vertical, sometimes present gentle slopes. 

 The strata are less strongly contorted in the lower parts 

 -than in the neighbourhood of the upper surface. They 

 are disjoined in certain parts by fissures or caverns ; 

 tney are traversed by clefts or faults inclined or vertical. 



All these deformations are the more varied in that they 

 are not similar on the opposite sides of the same band or 

 clay. 



Most of these phenomena are seen in Fig. i, 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 



