or THE EIO-TINTO MINES. 



261 



exist in the quartz-syenite ; and some of these could certainly be 

 worked to advantage by Cornish methods if the country were more 

 fully opened up by roads and railways : but I reserve for the present 

 all further reference to them. 



Fig. 9. — Slate ivith Kernels of Cohaltiferous Oxide of Manganese 

 near Bella Vista. 



4. Conclusion. 



A consideration of the facts here brought forward leads to the 

 following conclusions : — 



1. As to the Stratigrajphy of the District: — 



(a) The slates are of late Devonian age, but they include in some 

 places portions of still older clastic rocks, which have not yet been 

 recognized in the neighbourhood in situ. 



(h) The slates, in parts, like the well-known " Kupferschiefer '" of 

 Mansfeld, were originally highly pyritous and cupriferous; but 

 being interstratified with ordinary Posidonomya-sohists, they are 

 plainly much older. 



(c) After the slates had been deposited and upheaved, they were 

 cut through by great masses of syenite. As this syenite ranges 

 pretty nearly with the present strike of the slate, it is probable that 

 this latter had been folded into its chief synclines and anticlines 

 before the intrusion of the syenite. The slaty cleavage, too, which 

 now corresponds generally, but not invariably, with the bedding, was 

 also produced previous to this intrusion, but may have been since 

 increased. 



{d) Eoth slates and syenite have been penetrated by veins and 

 masses of diabase. In the slates the diabase often follows the 

 stratification for considerable distances and then cuts across at a 

 very oblique angle, the mean direction being E. to W. In the syenite 

 no such prevalent direction of the diabase veins is observable. 



(e) The porphyries are distinctly intrusive in the slates, and not 

 actually interstratified with them, although they often appear to be 

 so. A kind of selective metamorphism has, indeed, converted some 

 Q.J.G.S. No. 163. u 



