THE GEOLOaiCAL AGE OF CENTRAL AND WEST CORNWALL. 39 



occurrence is exactly what might be expected if the trappean 

 rocks were injected while the pressure of rock above was com- 

 paratively little, and the porphyries when it was very great ; in 

 other words the elvans are more decidedly plutonic than the 

 greenstones. 



Although the mechanical effects of the various intrusions are 

 so slight, the same cannot be said of the chemical effects. 

 These are everywhere visible, they differ in their nature in 

 different places, and the intensity and extent of the metamor- 

 phism seems to be somewhat proportioned to the extent of the 

 intruding mass. As to the different nature of the alterations 

 produced in the sedimentary rocks near the junctions, the gneiss 

 and spotted schist of the North Coast, near Portreath, and the 

 tourmaline schist of the Wheal Yor Valley and of the Hens- 

 barrow granite are good examples. As already observed, part 

 of the difference is no doubt due to the originally different 

 nature of the schists themselves, and part may be traced to the 

 subsequent circulations of Mineralized Waters through the 

 weakened and loosened junctions. Much limestone may have 

 been in this way removed by the action of acid (siliceous) waters. 

 This would account for many minor irregularities in the stratified 

 rocks — owing to the collapse of the unsupported beds on either 

 side. Many of the beds are highly silicified throughout in the 

 neighbourhood of the fissures, owing to the long-continued 

 action of such waters. 



With regard to the relative extent of the changes produced — 

 these are greatest in the neighbourhood of the granitic bosses, 

 and least in that of the elvans, which is quite in accordance with 

 the differences in bulk of the several intrusive masses. Much 

 more might be said on this interesting topic, but to do so, would 

 make this paper too long. 



Speaking generally, it may be said that as we go westward 

 the rocks met with are older and older. This is just what we 

 might expect from what is known of the mechanical movements 

 to which they have been subjected. 



Although, as a rule, the faults of Cornwall do not appear to 

 be of the first magnitude, yet some of them are of considerable 

 extent, and as they generally occur in series, the downthrows 

 being mostly in one direction, the final result is great. Thus 



