206 



Wilson — On Contortions and Faults, 



law of the inclinatioTi of the plane of the fault as follows : — " Sup- 

 pose til at in the diagram (Fig. 

 2) we have a portion of the 

 earth's crust, of which a b is 

 he surface, and c d a plane 

 icted on by some wide spread 

 force of expansion tending to 

 bulge upwards the part a, b, 

 0, D. If, then, a fracture take 

 place along the line e f, it is 

 obvious that the expanding force will, on the side of a c, have the 

 widest base c f to act upon, while it will have a proportionately- 

 less mass to move." Jukes then proceeds to discuss the junction 

 between two opposite faults, which produces what is often called 

 a " trough," of which his explanation only differs in some details 

 from my own ; only that my own is applied to all faults, and de- 

 pends on no hypothetical causes whatever. 



Similarly when I read manuals with a view to see what account 

 is given of contortions, I meet with nothing but general allusions to 

 " forces of disturbance," " unequal densities and pressures," and no 

 clear mechanical account of their origin. 



Lyell's account is that they may be due to lateral pressure, and 

 two ways of producing lateral pressure are indicated ; one by the 

 injection into fissures of molten, or the protrusion of solid rocks, and 

 the other by unequal degrees of subsidence arising from various 

 causes. 



Now, contortions and faults are, I think, readily explained when 

 one recollects — 



(1) That depressions and elevations take place over large areas. 



(2) That the surface of the earth is curved. 



(3) That rocks are compressible by foldings. 



(4) That rocks are not extensible or elastic. 



(5) That at great depths rocks are somewhat plastic through heat. 



For, consider a portion of the earth's sur- 

 face A, B, c, (Fig 3) and suppose it to be 

 an area of subsidence, or to sink graduall}" 

 to the position occupied by the dotted lines. 

 It is clear that to do so it must be laterally 

 compressed. Here is a source of power 

 for producing contortion, viz., the prodi- 

 gious weight of the mass, slowly sinking, 

 and crumpling up into curves and folds 

 that part of the area a, b, c which yields 

 most to lateral pressure. 



Contortions then are the inevitable result 

 of subsidence of a curved surface. 



Now, consider the re-elevation of a dis- 

 trict. The rocks have to expand so as to ^'^s- »• 

 occupy a larger area. How can this be accomplished ? Clearly by 



