78 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



extremity to the other, and therefore the transmitted pressure at one 

 extremity would nearly equal that impressed at the other. On the con- 

 trary, if the space between the hues ah and «,*,, represent the section 

 of the bed, it is manifest that the smallest number of straight lines 

 which could be drawn entirely within this space, so as to form a con- 

 tinued but broken line between a and h, would be considerable, and 

 that consequently the loss of transmitted pressure would be considerable. 

 The magnitude of the impressed pressure at a is limited by the power 

 belonging to the incumbent mass of resisting dislocation there; and 

 when the loss of pressure by transmission is so great, that there is no 

 longer sufficient force to cleave the mass into which the injected matter 

 is penetrating, the horizontal injection will cease. I think it very pro- 

 bable that the limits thus imposed on the extent of possible injection, 

 in the case of a thin bed like that just described, may be much nar- 

 rower than some geologists seem to have conceived. 



§. Effect of Joints in determining the Directions of Fissures. 



I liave stated (Introd. p. 11), that the investigations of Sect. I., are 

 not to be considered as applicable to a mass in Avhich the jointed 

 structure sliould prevail generally, because the cohesion of the mass 

 beiu"- in great measure, or altogether destroyed along the joints, the 

 fissures resulting from any external force, would of course be formed 

 along them. If, however, there should be two systems of joints existing 

 ])reviously to the action of the elevatory forces, in directions respectively 

 parallel and perpendicular to the general axis of elevation, it is evident 

 that the systems of fissures produced by this force, as well as all the 

 phenomena resulting from them, would be exactly the same as those 

 already described. If the direction of these systems should be only ap- 

 proximately parallel, and perpendicular to the axis of elevation, the same 

 would still be true as respects the distinctive characters of longitudinal 

 and transverse fissures, (see Art. 56). If, however, the directions of these 

 two systems of joints should not have approximately these relations 

 to that of the axis of elevation, or should not be nearly at right angles 

 to each other, systems of fissures will result different from those which 



