4 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



have been traced for four or five miles in the mining districts of this 

 country, and in some cases to the distance of eiglit or ten miles. 



^. Their depth appears to be uniformly greater than that to which 

 man has been able to penetrate. 



»;. The width of the fissures in that system of the two above mentioned 

 which contains the most productive and the most continuous mineral 

 veins, varies in general from a few inches to about 12 feet. In the 

 same vein the Avidth will frequently vary, and sometimes suddenly 

 along the same vertical line. In passing through a horizontal bed of 

 clay the fissure will be sometimes almost entirely closed; and the toad- 

 stone of Derbyshire produces the same effect, frequently closing the 

 fissure so effectually that it can only be traced through it by means of 

 small ramifying veins of calcareous spar. The average width however 

 does not appear at all to diminish as we descend*. The strata through 

 which the fissure penetrates generally form well defined though uneven 

 walls bounding it on either side, and perfectly firm and solid, except 

 where the strata themselves cease to be so. 



6. The width of the cross courses is frequently greater than that 

 above stated, and generally much more irregular. 



t. The fact of the strata in one wall of a fissure being higher than 

 the same strata in the opposite one, has been recognized by all miners 

 in some parts of almost every vein of consequence that has been ex- 

 plored, when existing in a distinctly stratified mass. This difference in 

 general does not exceed a few feet, though it has not unfrequently 

 been found to be many fathoms, in which case the vein of course coin- 

 cides with 2, fault. This is sometimes termed by miners the throw\ of 

 the vein. 



* In the mining district about Alston-moor there appears to be a few exceptions to this 

 rule, as well as to the assertion of the preceding paragraph {jQ, in what are termed gash veins. 

 These are comparatively wide at the top, and become gradually narrower as they descend, 

 till they appear to terminate. (See Forster's account of this district, p. 186.) They are pro- 

 bably rents the formation of which began at the surface, but are hardly worthy of notice as 

 exceptions to our general rules. 



t A thram is in fact a small fault. 



