THE GEOLOGICAL AGE OF CENTRAL AND WEST CORNWALL. 37 



sucli remarkable features In the coast line from Marazion to 

 Prussia Cove. 



The Ponsanooth beds have their own pre-granitic intrusive 

 rocks — which are especially well developed in the neighbourhood 

 of Penryn. 



At a period much later than the last of these intrusions, the 

 several granitic masses were thrust through the complex mass of 

 highly contorted and greatly altered stratified rocks — probably 

 by many partial movements, and even at several distinct dates. 

 These movements have been followed by the intrusion of the 

 porphyritic dyes known as " Elvans," which cut through the 

 granite also. Still later, the whole country has been very 

 extensively fissured — at ten or twelve distinct periods — and these 

 fissures have become filled with various mineral substances, often 

 in successive layers. Local metamorphism is often very evident, 

 not only in the neighbourhood of intrusive rocks, but also in 

 the vicinity of the fissure-veins but I merely refer to these 

 complicated phenomena here, in order to fix their places in the 

 sequence of time. Any description of them in detail would be 

 foreign to my present purpose. 



Conclusion. 



In the foregoing rapid sketch of the stratigraphy of Central 

 and West Cornwall, I have brought forward evidence to prove 

 that the country southward and westward of the Bodmin Grranite 

 is composed of no fewer than four distinct sets of stratified 

 rocks of very different ages, and not of two kinds only as shewn 

 on the Survey Maps ; and moreover, that the boundary line of 

 the Lower Silurian rocks on those maps is entirely and very 

 extensively inaccurate. I may add that the Survey Maps 

 themselves, rightly interpreted, very strongly support this 

 conclusion. 



If it be asked, how it is that this important discovery has 

 only now been made ? I reply that it would, I believe, have been 

 made long since, but for two most unfortunate assumptions. 



The early geological observers in Cornwall first assumed the 

 unity of all the rocks called by the miner "Killas," and then 

 accounted for every marked variation of dip and strike in this 

 killas by referring it to the intrusion of the granite. Thus, 

 most writers follow De la Beche in speaking of the killas 



