60 Mb HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



aUuded to, (Introd. ii. o, -n-,) presenting apparent liorhontal displacements 

 of the mass on one side of the unbroken veins, is not at first sight so 

 easily accounted for, since it can hardly be regarded perhaps as physi- 

 cally possible that any horizontal pressure can have acted on the mass 

 with sufficient intensity to produce an absolute displacement equal in 

 many instances to the apparent one. A very ingenious mode has, how- 

 ever, been suggested* of explaining phenomena of this kind, by referring 

 them to relative vertical movements of the masses in which the fissures 

 have been formed. It will not be difficult to convey an idea of the 

 manner in which this may be efFected. 



62. Let the annexed figure (1) represent a horizontal section at the 



^ (1) 



surface, of two veins wliich intersect, both being somewhat inclined to 

 vertical planes through AB, ED respectively. Now suppose the portion 

 of the mass bounded by the horizontal surface MN, and the nearly verti- 

 cal plane ABC (Fig. 2.)t of ^^^ ^^i"^ ^^' *° ^^ elevated (or the opposite 

 portion to subside), so that the surface 31' N' may be at a lower level than 

 MN. If this change be effected by a movement parallel to the plane A EC 

 of the vein AB, CE (Fig. 1.) will assume the position C'E (Fig. 2.); and 

 if EFG be a plane parallel to ABC, and intersecting the vein DCE 

 (Fig. 1.) in EG (Fig. 2.) CEG will be the plane of the vein in the sub- 

 sided mass, and it will no longer coincide with the plane DCC, the origi- 

 nal plane of the fissure DCE. If we now conceive the higher portion of 



* By the late M. Smidt. 



+ The same letters denote the same points of the mass in the diagrams (I), (2), (3), (4). 



