302. ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



West Pink, Tin lode. 

 Wheal Prudence, South lode. 

 Penhall'.s, North underliers. 

 Polberrow, South branches. 



,, Trevaunance lode. 



,, Pye's lode. 



Polgooth, Little Bound's lode. 



Class IV. — The Newer Elvans. 



The well-known elvan courses which cut through stratified 

 and unstratified rocks indiscriminately, appear to occupy lines of 

 fault as may be readily seen by carefully comparing the country 

 rocks on either side of them.* Often there is a difference of 

 hardness or of colour, occasionally of mineral character or of 

 strike, and like the lodes they sometimes include portions of the 

 country rock.f They may be regarded as fault-fissures with 

 granitic filling. The dips and strikes vary a great deal, and 

 there are often curves and branches on a small scale, but inmost 

 cases there is a general coincidence observable between the 

 direction of the elvans and of some one set of joints or lodes 

 in any particular district. The dips are usually from 40" to 60" 

 from the horizontal as stated by Mr. Henwood, this is consider- 

 ably less than the mean dip of the tin and copper lodes. 



Some able practical geologists have been much troubled to 

 account for the occurrence of elvans, considered as masses of 

 matter which have been forced from below into the solid 

 rock. Thus Capt. Charles Thomas says : — "referring to the 

 great elvan at Dolcoath, some geologists would have us believe 

 that this mass of elvan, several miles in length, of nearly a 

 uniform breadth of 60 feet, was thrown up at some period or 



* Mr. Thomas long ago saw this analogy between elvans, lodes, and cross- 

 courses. He says : — 



" It is very probable that at the opening of some of those chasms in which 

 the elvan courses are formed (if they have been formed in this way), or of some 

 of those fissures in which are the lodes, the ground may have been shifted in a 



similar way to what appears to have happened at the cross-courses some 



circumstances relative to the walls of the lodes bordering on the granite ground, 

 being on one side granite and on the other side killas, will further countenance 

 such an opinion." (Rep. in the Chacewater district, p. 23). 



f As fpr instance the •well-known Pentewan Elyan, 



