Jean-François Berger

“ There is another fact pretty well known to the Cornish miners, viz. that a change of the rock in which a vein runs, produces a change in the degree of richness, for instance, if a vein was in grauwacke, it would be impoverished by entering the granite, and vice versa.
It was formerly conceived, that the veins of copper belonged exclusively to the grauwacke, and that those of tin were chiefly confined to the granite; but there are now several copper veins worked in the primitive rock: it is however true, that they are near its junction with the grauwacke. ”

“ Though the cliffs at Stonehouse are of limestone, nevertheless Mount Edgecumbe, which is only separated from them by the Hamoaze, belongs to the grauwacke formation. We here find that rock passing from the state of a coarse argillaceous sandstone, of a reddish colour, not effervescing, and stained by oxide of iron, to that of a fine-grained grauwacke-slate, with red veins, giving it the appearance of ribbon jasper. It occurs in strata, dipping S.S.E. at an angle of about 65° or 70°, which is nearly the same as that of the limestone cliffs of Plymouth harbour. ”

“ The next formation in importance, which succeeds the grauwacke, is the serpentine. But although it may be seen for some miles before, in the form of large blocks scattered here and there in the fields, and forming part of the materials of the walls of the in closures, it is not upon this rock that the grauwacke immediately rests, but on a kind of talcose slate, of a greenish grey colour at the surface, tender and shining, soft to the touch, and evidently stratified: it forms a bed of small thickness, the boundaries of which it is difficult to determine. ”

“ The miners are affected in somewhat similar manner as the unfortunate criminals who are employed in roasting the ore in the arsenic and cobalt mines of Saxony, where they live almost continually in an atmosphere of arsenical vapours, and are seen to languish at the end of a few years. This poison is, no doubt, infinitely less powerful in Cornwall, but there are many other circumstances which combine with it to destroy the health of the workmen. ”

“ In the Isle of Wight, the tract of land to the north of the ridge of chalk, from Newport to Cowes, is but little elevated, being indeed almost flat: in Portland, the abruptness of the cliffs on all sides is rather considerable, and as far as I have been able to judge, pretty nearly the same, somewhat less perhaps to the south.
St. Aldham's Head, the most projecting part of the peninsula of Purbeck, lies exactly on the same parallel as the southern part of the Isle of Wight, and both belong to the same formation, as does also the Isle of Portland, which projects still further to the south. ”

“ A branch of the Fal flows through it, and the water, which on the heights near its source has been employed in washing porcelain earth, retaining the white colour of the earth, has at a distance the appearance of milk, which produces a very singular effect.
In proportion as we ascend, the grauwacke slate disappears, and in the neighbourhood of St. Stephen's church, it passes into the state of cornéenne, or of common grauwacke. It is stained with oxide of iron, and accompanied by numerous veins and pebbles of quartz. All these appearances indicate that we are approaching towards its limit. ”

“ The fracture of the rock is earthy, with many small specks and nodules of indurated clay. The outside of the rock is covered with a thin layer of calcareous spar. The specific gravity of the specimen I tried, was 2,052. The mass effervesces with acids, but the nodules of indurated clay do not.
This rock passes gradually to a bituminous shale. The first transition is to a slate-clay of a lighter or darker colour, the joints of which are covered with iron pyrites. It burns with a yellowish flame, giving out a sulphurous smell, and becomes afterwards of a light grey colour. ”

“ As a cap of eternal snow completely covers the upper part of the mountain, it is only by analogy we can say that the serpentine reaches to the top, it can however be traced to a very considerable height. ”

“ Let us imitate the example of the most skilful geologists, of Pallas, and of Saussure, who without ever losing sight for an instant of general views, did not cease to collect in the most patient and judicious manner, observations of detail, which if not in sufficient number to enable us to explain all appearances, have nevertheless the immense value of serving as a compass, and thus preventing us from making retrograde steps in our researches after truth. ”

“ The structure of the first is compact, dividing by natural joints into rhomboids or parallelepipeds: it is harder, not so fine in the grain and of a darker colour than the other variety. This last quality depends, no doubt, on its containing a greater proportion of iron. I believe that common grauwacke is a cornéenne trap of the French mineralogists. In Cornwall it is always found higher than the grauwacke slate: it may be supposed to have been precipitated more slowly, and under a less powerful pressure, whereby the mass has been allowed to contract, and assume a kind of crystallisation. ”

“ A blow of the hammer alone, upon some parts of the ore, produces a very powerful smell. It is possible, however, that this smell of garlic may in part be produced by antimony, for we know that this metal, even in a pure state, when strongly heated, gives out a vapour, the smell of which is very similar to that of arsenic. Whatever may be the respective effects of these various causes, it is certain that the miners never live to an advanced age: fifty-five is a very long life for them. ”

“ With regard to the nature of the granite itself, the crystals of felspar in it are numerous, very large, and often in an earthy state: the rock is crossed in many places by felspar veins of considerable length: black tourmaline, in large and small crystals, is also very common in the mass: taken in its whole extent, this granite is rough and uneven, furrowed on the surface, and may be said to bear very marked proofs of the hand of time. ”

“ These granite veins, whether they are found at the surface or at a greater or less depth, are in Devonshire and Cornwall, invariably directed from north to south, which is a direction opposite to that of the metallic veins, but quite conformable with the mode of deposition of the grauwacke on the mass of granite which forms the low mountain chain of Cornwall.
5. These subterraneous granite veins are rarely metalliferous, but when they do contain a metal, it is always tin, which is known to belong to the oldest formation, and which sometimes forms one of the component parts of the granite. ”

“ But whatever may be the age of these different parts of the vein, they are all necessarily posterior to the rock of the mountain in which the vein occurs, and in the present case, to the granite and the grauwacke, the only two formations of any extent that are met with in Cornwall and Devonshire.
As the same vein-stones are found in different formations of veins, we must therefore admit, that the same rocks have been held in solution at different periods.
Massive quartz sometimes forms alone the greatest part of the vein-stone of certain veins. ”

“ It is much stained by oxide of iron, which tarnishes the lustre of the calcareous nodules and veins contained in it.
Thorverton quarry is seven or eight miles north of Exeter. In going to it by Cowley Bridge, on the summit of a hill which overlooks Exeter, and is almost close to the town, the grauwacke is met with in situ. It is of a greyish colour, with very distinct and separate grains, breaking spontaneously into rhomboidal fragments; it is harder in some places than in others; when tender, it splits in the manner of slate, and assumes quite the character of that species of rock. ”

“ Mary Tavy, near the place where the roads join which lead from Plymouth to Oakhampton, and from Tavistock to Two Bridges, there is a bed of greenstone of some feet in thickness, in the grauwacke slate; it decomposes into a green earth. With regard to the grauwacke itself, the direction and inclination of its strata continue the same, it only contains more quartz as we approach St. Mary Tavy, and becomes at the same time less slaty. ”

“ Among the several formations [2] which the greater part of geologists recognize, (understanding by this term not only the nature of the rocks of a district, but likewise the general disposition of the strata, and their relation in point of position to the surrounding country) one of the most interesting undoubtedly is that of flint in chalk, which is subordinate to the secondary formation in the great and generally admitted division of rocks into primitive, transition, and secondary. ”

“ Between Rockenend and Blackgang Chine on the S.S.W. coast of the Isle of Wight, a land slip happened in 1799, the fragments of which cover a space of near half a mile in diameter.
As we walk along the cliffs, we see every where the surface of the soil rent by deep fissures; but a circumstance particularly remarkable is, that in this district, the decay seems to begin with the upper strata, which are gradually removed in succession. At Freshwatergate and at the Needles, those standing pyramids of chalk present us with a striking illustration of this supposed mode of decay. ”

“ Kirwan says, that although the oldest serpentine is not metalliferous, yet it contains magnetic iron, and veins of copper. [66]
It is easier to shew, that these characters are for the most part erroneous, than to substitute others more precise, and consequently better, in their stead: a negative truth is however in my opinion always of some value.
Monte Rosa is we know, next to Mont Blanc, the highest mountain in Europe; there being a difference of only ninety-six feet between them. All mineralogists and geologists agree that it is an assemblage of primitive mountains. ”

“ It is remarkable that in England, and also in Scotland, all veins of whatsoever age, or of whatsoever ore they may be composed, have nearly the same position, which would seem to indicate, that the cause to which they owe their existence, had acted in the same direction, though at different periods. ”