174; Mr. Hopkins's Reply to Dr. Boase's Remarks 



of the format'on of the existing dislocations subsequently to 

 the existence of the joints in the perfection in which they are 

 developed at present. 



I have also observed another fact in several places both in 

 this limestone district and the neighbouring coal district, of 

 great importance with respect to these theories, viz. that de- 

 viation, in several instances, from the regular parallel direc- 

 tions of the longitudinal and transverse fissures which, as I 

 have shown in my memoir, might be expected to accompany 

 ceriam partial elevations, such as I have observed them to be 

 associated with in the districts above mentioned. These facts 

 are strikingly in accordance with the theory I have investi- 

 gated, and as directly opposed to that which would assign 

 these dislocations to the previous structure of the mass. 



These observations agree also with those of Piofessor Phil- 

 lips as recently given in his Geology of Yorkshire. The prin- 

 cipal joints in that district have the same direction as in Der- 

 byshire, while the absence in general of any perfect coinci- 

 dence of lines of dislocation and of joints, affords conclusive 

 proof that the former cannot have been principally deter- 

 mined by the previous structure of the mass *. 



Among the dislocations of Derbyshire 1 include the great 

 characteristic mineral veins of the district. They may, in fact, 

 as I have elsewhere stated, be regarded as small faults, and uni- 

 formly in the vicinity of larger ones. There are also mineral 

 veins, some of which are as manifestly formed in open joints. 

 These differ from the former in no respect except in being of 

 much smaller dimensions. The vein-stuff is perfectly similai-, 

 and we are led, I think, almost necessarily, on inspection, to the 

 conclusion that it must have been deposited in the same man- 

 ner in both classes of veins. Now where veins are formed along 

 lines of faults, itsurely seems almost impossible not to conclude 

 that it must have been by some subsequent deposition or segre- 

 gation along the line of dislocation, and consequently, from 

 what I have above advanced, I consider it highly probable 

 that the veins in open joints were formed there in a similar 

 manner after the formation of the joints had commenced, and 

 probably during their gradual enlargement by the contraction 

 of the mass or other causes. The probability that open fissures 

 once existed in the places where these veins are now formed 



* The author of the work above referred to has attributed, in his theo- 

 retical speculations, more influence to the jointed structure in determining 

 lines of dislocation, than I conceive the facts alleged by him will justify. 

 At the time that part of his work was written, I am not aware that any 

 other physical cause of the laws of these phaenomena had been carefully 

 considered. 



