J66 ON THE MINERAL AND METALLIFEROUS [Ch. IX. 



when he attempts to offer an accurate definition, then he 

 learns that his notion of it is vague and objectionable. 



What is a vein ? There are many kinds of veins. Cer- 

 tainly ; but what is the nature of those veins occurring in 

 primary districts, which are metalliferous, and have been 

 explored in Cornwall by workings, many thousand miles in 

 length, and which are commonly known as true veins ? 

 Surely there can be no hesitation in answering this question, 

 since the subject has been so extensively examined. Werner 

 has, indeed, stated, and succeeding writers have adopted his 

 views, that " a true vein is understood to be the mineral 

 contents of a vertical or inclined fissure, nearly straight, and 

 of indefinite length and depth." Now this statement is 

 open to two rather serious objections : in the first place, it 

 cannot be demonstrated that these veins were originally 

 fissures ; and until this be done, it ought not to enter into 

 the terms of a definition : and, in the next place, no metalli- 

 ferous veins in Cornwall strictly answer to this description. 

 Mr. Carne must, in some degree, have been sensible of this 

 discrepancy, for when endeavouring to point out the differ- 

 ence between true and contemporaneous veins, he has re- 

 marked that " there are few veins which can be brought to 

 the test of all these marks ; and there are, probably, excep- 

 tions to some of them : some veins which are very short are, 

 perhaps, true veins ; and others, of considerable length and 

 width, and tolerably straight, may, possibly, be contem- 

 poraneous." * 



It is evident, therefore, that this account of veins is very 

 imperfect, and that a good definition is still a desideratum. 

 In order to accomplish this object, it is necessary, in this as 

 well as in every other branch of natural science, to obtain, in 

 the first place, numerous and accurate descriptions of veins. 

 So that, when we look around, and find that we do not 

 possess such data, not even, indeed, a single instance of a 



* Gcol. Trans, of Cornwall, vol. ii. p. 52. 



