Prof. Tyndall on the Conformation of the Alps. 261 



are in great part composed of sedimentary rocks which must 

 have required a sea to form them. Their present elevation above 

 the sea is due to one of those local changes in the shape of the 

 earth which have been of frequent occurrence throughout geo- 

 logic time, and which in some cases have depressed the land, 

 and in others caused the sea-bottom to protrude beyond its sur- 

 face. Considering the inelastic character of its materials, the 

 protuberance of the Alps could hardly have been pushed out 

 without dislocation and fracture ; and this conclusion gains in 

 probability when we consider the foldings, contortions, and even 

 reversals in position of the strata in many parts of the Alps. 

 Such changes in the position of beds which were once horizontal 

 could not have been effected without dislocation. Fissures 

 would be produced by these changes ; and such fissures, the ad- 

 vocates of the fracture theory contend, mark the positions of the 

 valleys of the Alps. 



Imagination is necessary to the man of science, and we could 

 not reason on our present subject without the power of presenting 

 mentally a picture of the earth's crust, cracked and fissured by 

 the forces which produced its upheaval. Imagination, however, 

 must be strictly checked by reason and by fact. That disloca- 

 tions occurred cannot, I think, be doubted, but that the valleys of 

 the Alps are thus formed is a conclusion not at all involved in 

 the admission of dislocations. I never met with a precise state- 

 ment of the manner in which the advocates of the fissure theory 

 suppose the forces to have acted, — whether they assume a general 

 elevation of the region, or a local elevation of distinct ridges : or 

 whether they assume local subsidences after a general elevation, 

 or whether they would superpose upon the general upheaval minor 

 and local upheavals. In the absence of any distinct statement, I 

 will assume the elevation to be general — that a swelling out of the 

 earth's crust occurred here, sufficient to place the most promi- 

 nent portions of the protuberance three miles above the sea-level. 

 To fix the ideas, let us consider a circular portion of the crust, say 

 one hundred miles in diameter, and let us suppose, in the first 

 instance, the circumference of this circle to remain fixed, and 

 that the elevation was confined to the space within it. The up- 

 heaval would throw the crust into a state of strain ; and if it 

 were inflexible, the strain must be relieved by fracture. Cre- 

 vasses would thus intersect the crust. Let us now inquire what 

 proportion the area of these open fissures is likely to bear to the 

 area of the unfissured crust. An approximate answer is all that is 

 here required ; for the problem is of such a character as to render 

 minute precision unnecessary. No one, I think, would affirm that 

 the area of the fissures would be r jV () th the area of the land. For 

 let us consider the strain upon a single line drawn over the sum- 



