32 Mr. Hopkins on the Structure of the 



zontal bed to the contiguous one. In these cases the general results will still be 

 accurately true, so far at least as we are now concerned with them. I have also 

 shown what modifications would be produced in the phsenomena by the existence 

 of planes or surfaces within the mass, irregular in position and of partial extent, 

 along which the cohesive power may be much smaller than in the immediately 

 contiguous portions of the mass. They may be termed surfaces of discontinuity, 

 and consist of irregular joints and cracks formed in the process of solidification, 

 or by partial disturbance previous to the general action of the elevatory force, 

 the effects of which are to be investigated. In such cases the lines of elevation 

 (the phsenomena with which we are here principally concerned), instead of being 

 straight or regularly curved, will be broken or zigzag lines or curves ; but their 

 general directions will be approximately the .same as if the mass were uniform. 

 Such will also be the case if the mass be regularly jomted in several directions. If 

 it be jointed in two directions only, the mean line of fracture will be made to 

 deviate more or less from the direction it would take if the mass were homogeneous, 

 towards that of the system of joints with which the fracture makes the least angle, 

 but still always tending to resume the direction it would have in the homogeneous 

 mass, whenever the continuity of the joints is interrupted. 



With respect to the elevatory force, it is assumed that it acted simultaneously 

 throughout the disturbed area, with an intensity which might approximate to uni- 

 formity for large portions of that area, while it might be greater at particular points 

 of it. 



2. I have called the force elevatory, because the general character of most dis- 

 turbed districts seems to indicate that the subterranean force, tending to elevate 

 the earth's crust, has predominated over its weight tending to depress it ; but if we 

 suppose the contrary to have been the case, and the effect to have been a depression 

 of the mass acted on, the phsenomena with which we are principally concerned, 

 the lines of dislocation, will follow the same law as if the mass had been elevated. 

 It is only necessary that the resultant force at each point should generally be 

 nearly vertical, i. e. nearly perpendicular to the upper and lower surfaces of the 

 mass in its undisturbed position. I shall always speak of it, however, as elevatory. 



.3. We are not directly concerned in this theory with the physical cause to which 

 the existence of an elevatory force may be due. Our object is to account for the 

 varied phsenomena of elevation, by referring them to some simple and general 

 mechanical action ; and, in so doing, to consider how far we may be able to define 

 the nature of that action ; whether it may have been general or local ; and whether 

 the resulting effects may be attributable to many repeated efforts, or to few. To 

 give the greater definiteness, however, to our conception of the mechanical 



