248 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 1, 



coarser shales and grits, which, by the aid of plant-fragments in the 

 latter, are ranked as Carboniferous. 



The actual junction with the granite is concealed by a covering of 

 detrital matter, and similar beds occur irregularly on the hill-sides 

 throughout the district. This deposit is in all cases unrolled, and 

 evidently local, the result of some action of brief duration. 



The Devonian rocks dip at an angle of about 40° away from the 

 granite, or rather from a point in advance of the latter, so that the 

 district forms a dome of elevation, having the high ground of Cadon- 

 barrow (slate) for its centre, and connected wdth the granite by an 

 anticlinal line : one portion of the dome dips into the sea, another 

 towards the south in the great Delabole quarries, a third towards the 

 north, and the other is held up by a connecting line towards the 

 granite at Roughtor*. 



The strike of the beds, in a large curve round the granite, not only 

 prevails in the slate-rocks, but affects the masses of trap, which, as 

 the Ordnance Geological Map shows, are uniformly elongated in the 

 same direction. The small patches of calcareous matter, and even 

 the fissile slates, follow the same rule. 



The small dykes of trap are ordinary hard greenstone ; the larger 

 masses are soft chocolate-coloured and purple masses, occasionally 

 vesicular. 



Full information on the general geology of these trappean rocks, 

 and of the whole district, will be found by consulting Professor 

 Sedgwick's Memoir, iu the fifth volume of the Society's Transactions ; 

 Sir H. De la Beche's Report; and a paper by the writer, on the 

 geology of the Tintagel district, published in the Report of the Royal 

 Geological Society of Cornwall for 1847. 



The Devonian rocks here are traversed by siliceous bands in the 

 form of veins of coarse quartz. These are subordinate quartzose 

 jjortions of the slate-rocks ; not cross-courses or strings, but meta- 

 morphic conditions accompanying fissures iu the line of the bedding 

 and strike, and attended with the seg-regation or addition of various 

 mmerals. They were produced by a cause affecting apparently 

 the whole mass. The veins are variable in character as regards the 

 admixtures present with the quartz. Trappean matter is often 

 visible, usually mica, rarely pyrites. In some places the quartz is 

 much intersected by ferruginous partings and hollows ; these contain 

 " gossan," varying in colour from light pink to dark red and brown. 

 It is these gossaniferous portions, in the A-icinity of trappean matter, 

 which have been found to be auriferous. 



In the summer of 1852, the description of gold-rocks in Cali- 

 fornia led me to examine very cursorily similar rock-formations in 

 North Cornwall. From a portion of a quartz-vein at Davidstowe I 

 then obtained a trace of gold, and reported the fact to the Geological 

 Society of Cornwall. It is duly recorded in the published Report of 

 that Society for the year 1852. I mention this merely to show that 

 the discovery (if such it may be termed) is due to our science, and 

 not to hap-hazard. 



* See Report of Corinvall Geol. Soc. 1847, p. 8. 



