80 SIXTH REPORT — 183G. 



the directions and positions of the primary fissures as determined by 

 a regular structure of the mass (such as the jointed or laminated struc- 

 ture) superinduced previously to its elevation. Mr. Hopkins thought it 

 highly probable however that if such were the case the dislocations 

 would be much more numerous in any disturbed district, and much less 

 continuous than they are observed to be. He wished particulai-Jy however 

 to impress on the minds of geologists that the claims of the two theories, 

 one of which would assign the directions of the lines of dislocation to the 

 mode of action of the dislocating force (as explained in his memoir), and 

 the other to the previous constitution of the dislocated mass, must be 

 ultimately decided by observation ; and to enable observers to do this, 

 he begged to direct their attention to two or three points in particular, 

 which might, probably, in many cases, decide the question. 



1 . If the lines of dislocation which we obsen'e in the superficial por- 

 tion of the earth's crust were determined by the jointed structure which 

 we now observe in that portion, there must manifestly be, not an ap- 

 proximate, but an accurate coincidence of the joints and dislocations . 

 Wherever such is not the case, we have an indubitable proof that the 

 joints in the vpper portion of the dislocated mass could not have been so 

 far developed as to exercise any material influence on the directions of 

 dislocation. 



2. It may be conceived however that the lower portion of the mass 

 may have been so far jointed at the period of dislocation as to determine 

 the directions of fracture in the upper part. To determine the truth of 

 this hypothesis, the directions of the joints in the primitive rocks should 

 be carefully examined at points nearest to the observed systems of 

 dislocation, to ascertain how far this accuracy of coincidence or the 

 absence of it can be established. The want of it must necessarily 

 be conclusive, while its existence is inconclusive evidence as to the 

 point in question. Further tests must be sought for by examining 

 the directions of the lines of fracture in the proposed district; whether 

 for instance they are related to an axis or to a centre of elevation, 

 or whether they present distinct local deviations from the general 

 law of the district, connected with any peculiar local geological con- 

 formation. If such deviations be found, it must then be considered 

 how far they are inconsistent with the theory Mr. Hopkins has investi- 

 gated, or with the general laws which careful observation may hereafter 

 establish respecting the directions of well- developed systems of joints. 

 It is by these or similar observations, and not by any preconceived no- 

 tions as to the constitution of the dislocated mass, that the question at 

 issue must be decided. 



Mr. Hopkins elucidated these obsen'ations by a reference to the 

 limestone, grit, and coal districts of Derbyshire and Nottinghamshire, 

 in which two general systems of dislocation are well developed, the one 

 being N. and S., the other E. and W., but presenting some local devi- 

 ations from this law curiously in accordance with his theory. In the 

 same districts (more particularly in the limestone) there exist two ex- 

 tremely well defined systems of joints nearly at right angles to each 

 other, and running very nearly magnetic N. and S. and E. and W. 



