John Josias Conybeare

“ The nearest point at which granite occurs must be as much as seven miles distant: we were therefore not surprised at the total absence of those large granitic and porphyritic dykes which, whether they are connected with or independent of that primitive rock, are certainly of much more frequent occurrence in its vicinity than elsewhere. ”

“ In some parts the compact grauwacke was wanting for a considerable distance, in such cases the forms which the slate had assumed were rather angular than curvilinear. In the sections of those which have been called saddle-shaped strata, we observed usually that the dip was more precipitous on the western side. In neither variety of the rock could we discover any traces of organic remains, nor could we perceive any imbedded fragments that should indicate their having been formed from the debris of an earlier rock. ”

“ Although these singular contortions are not entirely confined to the dunstone and shillat of Devonshire, they yet occur much more constantly in that rock than in the metalliferous schist of Cornwall. In travelling westward we appeared to lose them gradually. At Bosscastle where the rock evidently approaches nearer to the character of killas, (as it is termed by the miner) they still appear pretty frequently: at St. Agnes they are much rarer and less capricious. ”

“ The average height of the cliffs, as far as we could judge by the eye, is 130 or 140 feet; they are traversed in many places by steep ravines running from north to south, and numerous outlying masses of rock shew themselves above the sea at a small distance from low-water mark, a character uniformly presented by the stratified rocks along the whole of the Northern coast. ”

“ The lodes are principally worked for tin, though copper is occasionally raised. The prevailing rock is killas, and the nearest point at which granite has been noticed is in the Gwennap cluster of mines about six miles distant. ”

“ A considerable part of the tin of Cornwall is obtained from the killas, and the grey ore of cobalt has repeatedly been found in it. All these minerals have usually been considered as characteristic of the earlier rocks, and none of them are, I believe, stated by the writers of our most esteemed mineralogical systems to occur in that to which they assign the name of grauwacke [5] . ”

“ Having observed in the walls of the neighbourhood several specimens of those porphyritic and granitic rocks, which are distinguished in the country by the name of elvans, we were induced to enquire more particularly into their Geological relations, and the frequency of small promontories on the coast afforded us a favourable opportunity of ascertaining their position in several spots. ”

“ As we are always accustomed to regard terms of science as retaining (where the contrary is not expressly stated) the precise sense in which they have hitherto been uniformly received, the use of this expression is perhaps not strictly correct. ”

“ The latter (the Killas of Cornwall) has generally the silky character ascribed to clay slate. [4] It is traversed almost throughout its whole extent by frequent veins, or rather dykes, of a porphyritic rock, which forms one variety of the Elvan of the Cornish miners, and which does not occur either in the dunstone or shillat. ”