ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 307 



Class I. — The Oldest Elvan-courses. 

 These are heaved by the oldest tin-lodes, as in the case of 

 the " little elvan " at Polgooth, which is heaved by St. Martin's 

 lode. Probably the elvan at Foxhole, near Nanpean, and other 

 elvans of the St. Austell granite district, having a highly f elsitic 

 filling, and sometimes a little oxide of tin in the joints and 

 shrinkage cracks, are of this age. Similar felsitic elvans may 

 be traced to small distances into the granitic masses wherever 

 china clay abounds. A few, too, are known in the killas, as at 

 Newham near Truro, and Trelaver Downs in St. Dennis,* but 

 usually the elvans in the slate are much more distinctly 

 porphyritic in character. 



CONCLTJDING EbmARKS. 



It appears to the writer that the facts set forth in the fore- 

 going chapter suggest the following as " a working hypothesis 

 which, if it does not explain every fact, is inconsistent with 

 none."* 



1. The stratigraphic relations of the sedimentary rocks of 

 the West of England Mining District were essentially the same 

 as they are now before the granitic eruptions took place • 

 carboniferous rocks resting on Devonians in the eastern part of 

 the region ; Devonians resting upon Ordovicians, and these upon 

 still older rocks in the West. It was already a very ancient 

 land which had been elevated and depressed, crumpled and 

 contorted, at many different periods ; furthermore, each stratified 

 series seems to have been subject to the intrusion of sheets or 

 dykes of eruptive rock, to as to include thick beds of lava or of 

 volcanic ash. 



2. The successive elevations had been followed by 

 extensive denudation, so that in many cases the newer series 

 reposed unconformably upon the older ones below it. 



3. The pre-granitic eruptive rocks seem to have been all 

 basic, pyroxenic, or amphibolic, rarely if ever trachytic, and, 

 locally, there was much magnetite and olivine present. Leaving 

 out of consideration the Lizard district, the " serpentines " of 



*See Trans. Eoy. Geol. Soc. Corn., IX,- p. 226. 

 *Clodcl. Story of Creation, Introductory, p. 3. 



