Mn HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



67 



tl.e previous section, that the formation of one or both of the fissures 

 may have been either contemporaneous with, or anterior to that move- 

 ment of the mass which produced these displacements; and consequently 

 the existence of the heave in the one or the other of two intersectin; 

 vems, can afford no test of their relative ages. In cases, however, where 

 several veins are foimd to have been heaved in the immediate vicinity 

 of each other (as in some of the Cornish veins) indications may be 

 obtamed of their relative ages from the phenomena they exhibit, assuming 

 them to have been produced in the manner just supposed. 



71. It has been stated (Introd. „, p. 4), that the fissure of a vein is 

 frequeirtly almost entirely closed in passing through a thin stratum of 

 clay. This fact may, I conceive, be easily accounted for from the o-reater 

 extensMity, and less elasticity of this stratum, as compared with the 

 masses with which it is interstratified. The former quality would allow 

 It to remain unbroken, with an extension which the general mass could 

 not but yield to, or if broken, it would from the latter property have 

 little tendency to recede to its original extent. 



72. It is not my intention to enter into any discussion on the 

 mode in which the fissures of mineral veins have been filled*; but I would 

 remark, that the frequent occurrence of the fact above mentioned seems 

 equally unfavorable to the hypothesis of this process having taken place 

 by superficial agency, or by any species of injection from beneath The 

 difficulty, however, assumes a far more formidable character when con 

 sidered with reference to the toadstone of Derbyshire, which, as I have 

 already stated (Introd. ii. r,.), produces the same effect, in nearly destroying 

 the continuity of the fissure, as the clay beds above mentioned But 

 in this case, instead of a bed of a few inches in thickness, we find 

 a bed of toadstone of from ten to forty fathoms, through which the 

 vein can sometimes be traced only by mere threads of calcareous 

 spar. How then can we conceive the upper part of such a fissure 



• I do not here allude merely to the process by which the v,hcral vein properly so 

 called, (see p. 2.) has been deposited, bat that by which the whole fissure may have been 

 filled wth the van stuff y,hich now occupies it. The fissure may be several feet wide, while 

 the mmeral vem is not an inch in width. 



