THE MINING DISTRICTS OF CORNWALL. 



335 



rocks which they traverse ; but they seldom penetrate between the 

 cleavage-planes of slates*. 



In slate they generally consist of a compact felspathic or quartzo- 

 felspathic base containing crystals of felspar and crystalline or gum- 

 like patches of quartz. When enclosed in granite, a similar base 

 prevails, mica and schorl are frequently present, and porphyriti- 

 cally imbedded crystals are numerous, but the rock is generally finer- 

 grained than when it is in slate. In both rocks, however, it is 

 usually coarser and more porphyritic near the middle of the dyke 

 than towards its sides. 



Elvans are traversed in all directions by joints dividing them into 

 irregularly shaped blocks ; in some cases these are filled with schorl, 

 while in others the filling material is a ferruginous or felspathic 

 clay. 



Throughout the principal mining districts of Cornwall the gene- 

 ral bearing of the elvan-courses is a few degrees north of east, and 

 they are therefore approximately parallel with the majority of the 

 most productive tin and copper lodes. In other parts of the county 

 elvans are sometimes found running nearly Dorth and south, thus 

 nearly coinciding in direction with the cross veins occasionally 

 yielding lead and iron ores. 



The following Table gives the composition of four different varieties 

 of Cornish el van ; but of these the first three only can be regarded as 

 true elvans. I. is coarse-grained and highly porphyritic ; II. is 

 much less coarse in grain ; III. has a conchoidal fracture and is 

 almost as compact as chert ; IV. contains an unusual amount of 

 brown mica, and in other respect differs materially from ordinary 

 elvans. 



Table showing the Composition of Four Varieties of Cornish EJvan. 



Water { hjgroinetric 



I. 



•11 



•49 



72-51 



13-31 



3-87 



trace 



•62 



•60 



1-52 



6-65 



•43 



trace 



II. 



•26 



203 



72-82 



1512 



trace 



1-75 



trace 



•52 



106 



6-25 



•51 



III. 



IV. 



•43 



1-27 



71-46 



15-38 



2-27 



•30 



trace 



•47 



•22 



5 51 



2-79 



•34 

 6-11 



47-35 



20-60 

 1-60 

 3-10 



trace 

 4-72 

 612 

 6-29 

 3-58 



trace 



Silica 



Alumina 



Ferrous oxide 







Lime 



Magnesia oxide 



Potassa 



Soda 



Fluorine 





10011 



100-32 



100-10 



99-81 



Specific gravity 



2-62 



2-64 



2-65 



2-70 



1 



* W. J. Henwood. Address delivered at Meeting of the Boyal Institution ot 

 Cornwall, May 1870. 



Q, J. G. 8. No. 123. 2 a 



