146 ORIGIN AND DEVELOPMENT OF OEE DEPOSITS. 



probable that no single dislocation is accompanied by more than 

 100 fathoms of vertical displacement, while many only produce 

 displacements of a few feet or even of a few inches. Nevertheless 

 the absolute number of faults belonging to each group is usually 

 so great that their cumulative effect is very considerable. Thus, 

 between Portreath and Perranporth on the north coast, a distance 

 of about nine miles, no fewer than thirteen considerable north 

 and south faults (cross-courses) are known, besides many minor 

 ones, nearly all of which have their down-throws to the east- 

 ward. Consequently, the geological horizon of the Portreath 

 rocks is considerably below that of the rocks at Perranporth. 

 One of these faults, the most easterly of the group, is known to 

 Occasion about 600-ft. of vertical displacement, and those which 

 are seen at Porthtowan, Tobban Cove, and Cayack Cove are 

 little less extensive. The total vertical displacement in the nine 

 miles referred to cannot be less than one mile, and may be as 

 much as two miles. 



Going east towards Padstow, north and south faults with 

 easterly down-throw are still frequently met with, and the total 

 vertical displacement between Portreath and Padstow will be 

 very moderately estimated at 15,000-ft., or say three miles.* 



The faults known as cross -courses not only cross the principal 

 mineral veins, but also for the most part cross the strike of 

 the rocks; they are sometimes spoken of as "dip" faults, but 

 better as ''cross-courses," while those that run with the strike 

 are called "strike" faults. This class includes most of the 

 principal east and west lodes of the country, and as already 

 remarked, it is not often that we are able to determine the 

 absolute amount of downthrow in such faults, although it is 

 probably as great in its totality as that of the cross-courses in 

 equal spaces. 



Surface indications of faults. That the surface sculpturing 

 of a country depends greatly upon the varying character of its 

 rocks is a geological truism. The effects of jointing have been 

 already referred to with some amount of detail. I propose here 

 to give a few further illustrations as regards faulting, drawn 



* See the Author's Geol. age of Central and West Cornwall. Journal 

 Eoyal Institution of Cornwall, viii, 1, 1881. 



