288 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



(and my own more recent observations agree with his) they 

 sometimes dip away from the nearest granite, and sometimes in 

 intermediate directions, but never directly towards it, and they 

 are thought not to occur in the granite. Their breadth rarely 

 exceeds one foot and is sometimes not more than two or three 

 inches. 



The amount of faulting produced by slides seldom exceeds 

 a few feet, so that their mechanical effects on the strata are 

 usually not great. The faulting due to the newest flucans, those 

 belonging to this group, is perhaps usually greater than that 

 due to the slides — the greatest of them however, that which 

 heaves all the veins at Polgooth, only produces a vertical 

 displacement of about 19 feet.* 



In connexion with the slides proper we may here consider 

 the " slidy ground " which has been met with locally in many 

 parts of Cornwall. The so-called " Q-reat Flucan " which 

 traverses the Wheal Eliza lodes between St. Austell and Par 

 is an example of such " slidy ground." Its width has been 

 variously stated at fifty, eighty, and one hundred fathoms. 

 Good examples of " slidy ground " were met with also at Wheal 

 Trelawney, Wheal Mary Ann, and Herodsfoot, all near 

 Liskeard. At Wheal Mary Ann, a broad band of broken 

 ground which runs nearly E.W. and dips to the south, cuts off 

 the lode completely in the South part of the mine after twisting 

 it out of its course. This " slidy ground " nearly a hundred 

 fathoms wide appears to be newer (i.e. its slidy character 

 appears to have developed more recently) than the flucan in the 

 same mine, which also intersects the lead lode.f 



At Herodsfoot the " slidy ground " dips northward and also 

 cuts off the lode. The nature of this "broken" or "slidy ground" 

 met with in connection with the lead lodes of this district is very 



remarkable and well worthy of attention The whole 



strata seem broken up by a succession of disturbances of a 

 nature between cross-courses and slides This broken 



* In the case of what has been termed "slidy ground" the cumulative 

 effect may be very much greater than is here stated. As a rule I believe the 

 amount of vertical displacement increases with the increase of inclination of a 

 fault from the hoi'izontal — as might indeed be expected. 



t Mining and Smelting Magazine, 11, 222. 



