Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



65 



be determined by the constitution of the upper mass, or some circum- 

 stance not immediately depending on the position of DC. In such a case 

 therefore, tliere might be an apparent horizontal shift of any magnitude' 

 If, hoAvever, a certain force, arising from cohesion and friction, should 

 oppose a relative horizontal movement of the upper and lower portions 

 of the mass, a hmit will be imposed on the extent of the apparent shift 

 for It IS obvious that this force must be called into action in the 

 formation of CE, (in the progressive formation upwards of the fissure) 

 by the opposite motions of the upper surface of the lower mass, and 

 lower one of the upper mass between C and C, and in no other part 

 Consequently, if the resistance at C to the formation of a fissure in the 

 upper mass, together with the lateral force just mentioned, be greater 

 than the resistance to the continuation of the fissure from C towards 

 E', the former fissure cannot be formed in preference to the latter 

 and thus a limit will be imposed on the distance CC. It is easy how' 

 ever, to conceive, from the known constitution of the beds which Appear 

 to give rise to phenomena of this kind, that this distance may be suf 

 ficient to account very easily for all such appearances of displacement 

 as we are now considering. 



68 If we conceive the figure in page 64 to represent a horizontal 

 instead of a vertical section of the mass, and cd to represent a fissure 

 then, if a fissure BCCE be propagated across it, it is manifest that 

 considerations exactly similar to the above would enable us to account 

 for the apparent displacement CC in this as well as in the former 

 case, and it appears higlily probable that such appearances may have 

 been not unfrequently thus produced. We may also observe that if 

 the fissure cd has not been completely filled, and its sides again cemented 

 together, the movements of the masses on opposite sides of it will be 

 in a certain degree independent of each other, so that a fissure DC 

 propagated so as to meet cd at C, might be continued on the other 

 side of cd, from a point C quite remote from C. In such case DC 

 would appear to terminate at C, and this, in fact, {DC being a smaU, 

 and cd a large vein) is not of unfrequent occurrence. 



69. There is also another manner somewhat different from the above, 

 in which an apparent displacement of a fissure may be produced It 

 Vol. VI. Part I. i 



