64 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



are formed is distinctly stratified. In such cases therefore, by comparing 

 our observed and computed values of C'C, we might obtain very ac- 

 curate tests of the truth of the explanation which has been given of 

 these phenomena. 



66. The value of the explanation which has been given above of 

 the phenomena we are now considering, consists in the substitution of 

 vertical for horizontal movements, and therefore depends on the approxi- 

 mate verticality of the unbroken vein, parallel to the plane of which 

 the motion is assumed to take place. It not unfrequently happens, 

 however, that a horizontal displacement of a vertical vein takes place 

 at the thin horizontal beds of moist clay, of which so considerable 

 a number is found interstratified with the mountain limestone. The 

 .slimy nature of these beds undoubtedly affords a great facility for a 

 relative movement of the masses respectively above and below them ; 

 and therefore where the displacement is small, there seems no difficulty 

 in accounting for it on the supposition of this relative motion. In other 

 cases a more probable cause may be found in the following considera- 

 tions. 



67. In the annexed figure let ccf represent a thin stratum of clay, 



of such a nature as to give a considerable facility to a relative horizontal 

 motion of the masses above and below it, and suppose a fissin-e to have 

 been propagated upwards by the action of horizontal tensions, from 

 D to C. If there were no cohesion whatever between the upper and 

 lower divisions of the mass, it is manifest that the position of DC 

 would not in any degree influence the position of a fissure C'E, which 

 might be produced in the same manner and at the same time in the 

 upper portion of the mass, and consequently the point C would then 



