286 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



been wanting,* yet, we find that in a large majority of cases of 

 intersection, displacements have been actually observed-! 



As each new set of dislocations tends to modify, and to a 

 certain extent obscure all those which have preceded it, the ideal 

 reconstruction of this ancient district will perhaps be best 

 followed by working backwards from the most recent to the 

 most ancient fissures. We will therefore commence with those 

 which appear to traverse the alluvial or at any rate the 

 superficial deposits. 



Class XV. — Alluvial Faults. 



Under this head it will be convenient to refer all movements 

 of the strata which have occurred since the formation of the 

 tin-gravels and other more recent (post pliocene ?) detrital valley 

 deposits. So far as is yet known they are not very numerous 

 and of but small extent, but they are important as shewing that 

 movements have taken place in comparatively recent times. | It 



* Mr. Thomas says : — " As tlie changfes of strata accompanying the lodes 

 were not the objects of search with the miners, there has not been so much notice 

 taken of them as to furnish any useful data towards the determination of this 

 question (displacements of strata occasioned by lodes as distinguished from cross- 

 courses), and in all probability we should not have learned anything satisfactory 

 about heaves, had the tin and copper and other metals been contained in the 

 cross-courses and not in the veins which are intersected by them." Report on 

 the Chacewater Mining District, p. 24. 



Again, Mr. Carne says : — " It is not at all strange that so few instances have 

 been met with of the intersection of cross-courses by cross-flucans and slides ; for 

 however frequently these may occur they are not likely to be discovered except in 

 adits which may be driven in the same direction, or unless they happen just at 

 their intersections of metalliferous veins ; for in general these cross-veins have 

 nothing valuable themselves to make them worthy of pursuit." Trans. E. Geol. 

 Soc. Corn. II, p. 111. 



t Mr. Henwood says : — " Of 272 lodes traversed by cross-veins in different 

 parts of Cornwall, 57 or 20°/o are intersected but not (heaved) displaced." As 

 the " average displacement " of the remaining 80''/o throughout the County is 

 close on 16 feet, it is probable that many of the 57 so-called intersections, without 

 displacements, are really accompanied by small displacements which would 

 scarcely be noticed in the intersections of lodes several feet wide, bounded in 

 many cases by capels or walls of not very distinct mineral character." 



;j:Examples of such faults are given by Mr. S. R. Pattison (Trans. Roy, Geo. 

 Soc. Corn., VII, p. 3(i) and by myself (Report Min. Assoc, of Cornwall, &c., 1872, 

 p. 70). I may here mention that in some mines near St. Austell I have known 

 the bimber caps of levels, not more than thirty inches wide at top, to be 

 gradually crushed together — sometimes after being placed only a few weeks and 

 when timbers 10 inches square were used for the said cap-pieces. 



