40 THE GEOLOGICAL AGE OF CENTRAL AND WEST CORNWALL. 



between Perranporth and Portreath, a distance of nine miles, no 

 fewer tlian 13 considerable north and south faults (cross-courses) 

 are known, besides many minor ones, nearly all of which have 

 their downthrows to the eastward ; consequently, the geological 

 horizon of the Portreath rocks is considerably below that of the 

 rocks at Perranporth. 



One of the faults, the most eastern of the series, is known to 

 produce 600 feet of vertical displacement, and those which occur 

 at Porthtowan, Tobban -Cove, and Grayack 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. 



Groing east towards Padstow, these N.S. faults with eastern 

 downthrows are still met with very frequently, and the total 

 vertical displacement between Portreath and Padstow will be 

 very moderately estimated at 15,000 feet. 



The existence of so many eastern downthrows suggests that 

 the western land was at one time the highest. It is now lower 

 by several hundred feet. This would seem to indicate a greater 

 amount of denudation westward, and a consequent exposure of 

 the older strata as we see them in fact. That the original 

 deposits were thicker westward is indicated by Mr. Sorby's 

 experiments on the fluid-cavities of the Cornish granites. Mr, 

 Sorby's conclusion was, that the granite of St. Austell solidified 

 under a pressure equal to that of 32,400 feet of rock, while in 

 the case of Ding Dong Mine, near Penzance, the pressure was 

 equal to 63,600 feet. Carn Menelez being in an intermediate 

 position, the pressure also was probably between these limits, 

 or say 50,000 feet, which would agree very well with the estimate 

 which I have made of the total thickness of the stratified rocks, 

 viz: 46,000 feet.*' 



The actual vertical pressure of those portions now existing 

 was probably never so much as this, as one set became crumpled 



* This is for the thickness of the rocks between Lostwithiel and Ponsanooth, 



viz : — 



FoweyBeds 10,000 



LadockBeds 1,000 



Lower Silurians 23,000 



Ponsanooth Beds 12,000 



46,000 



