252 Rer. 0. Fisher — On the Formation of Mountains. 



itself expanding, or else by the neighbouring rocks shrinking in 

 towards it. I have shown in my paper on the elevation of moun- 

 tains that an enormous horizontal pressure would result, before 

 which the layer in question, if incompressible, must give way, and 

 its position be disturbed. 



But would the horizontal pressure elevate it into an arch or dome ? 

 It appears to me more probable that, acting on a layer many times 

 wider than deep, it would break it up into a series of synclinals and 

 anticlinals interspersed with faults, and not cause it to rise in 

 one unbroken arch at all. The pressure of the rocks below is not 

 really capable of doing more than sustaining the original layer in 

 its original position, and, if that becomes weighted by the addition 

 of deposit, the tendency will be towards depression, and not eleva- 

 tion, so that lateral pressure would in the first instance cause it to 

 bulge downwards and not upwards. I think it likely that, the layer 

 once disturbed, the subjacent superheated rocks might rise into the 

 anticlinals, and if so I should expect that the synclinals would 

 equally sink down into those rocks. 



Let us suppose AB CD tohe a, layer of rock of unit of thickness, 

 length I, and depth k, and first that it would expand to Ab G d, so that 

 Ah = AB (1 -\- e), where e is a very small fraction, but that, on 

 account of the abutments at J. C and B D being immovable, it is 

 forced to assume some new form, as for instance that given in the 

 figure (or any other). Call A B the datum level. Now the first 

 thing which is evident is that the wavy lines AB and CD will be 

 shorter than the straight lines A h and Cd respectively, because the 

 stratum is all along^ subject to longitudinal compression, not only 

 at first, but also finally, because their weight will compress the anti- 

 clinals, and the inertia of the subjacent rocks will resist the synclinals 

 being forced down. Hence it is clear that the surface line cannot 

 be longer than I {1 -\- e), but will be, on the other hand, rather shorter 

 and the layer on an average thicker than before compression. 



Let us now seek for some simple laws which must govern the 

 disturbed strata in spite of the confusion which appears to reign 

 among them. Let a, a, etc., be the areas formed by the upper curved 

 line above A B, and h, h, etc., the areas formed by the same line 

 below A B. It is not necessary that the a's should be equal to one 

 another, nor yet the &'s. They are used simply to designate the 

 areas in respect of their positions. 



