Jean-François Berger

“ In considering the steatite, a mineral which still more frequently than the diallage and jade accompanies the serpentine, I shall adopt the same method I have pursued in treating of these last substances.
Steatite is found not only intimately combined with serpentine, but frequently in masses forming subordinate beds in that rock; such are the soap-rocks at the Lizard. ”

“ Between Ramsay and Aire-Point I saw in some heaps of stones a number of pieces and blocks of greywacke, passing from large grained into a small grained texture. The basis is greyish, the imbedded fragments are mostly nodules of white quartz with scraps of a dull and black sort of slate, fusible into a whitish enamel, and abraded plates or broad spangles of mica. The cavities that receive the nodules of quartz when freed from them, appear smooth and even. The pieces of slate turn sometimes clayey, micaceous, and of a brown ferruginous colour. ”

“ 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. ”

“ When, the thickness of the strata bears a more equal proportion to the length or respective distance of the dip-joints, the natural divisions will come to a cubical form. If on the contrary, the structure be thin-slaty, the rocks will rise spontaneously under the form either of flags or slates. Whatever may be the shape of these natural blocks, their two contiguous sides are constantly coated with the predominating ingredient of the rock itself in its present state. Thus if the rock be quartzose, the coatings of the dip-joints will be veins of pure white quartz ”

“ Among the substances that are combined with the different metals of which the mineral deposits of Cornwall are composed, there are two which merit particular notice, both on account of their very general occurrence, and also because they appear in certain circumstances to act considerably upon the rocks with which they are in contact, altering their texture, and disintegrating them, as if they had been exposed to acid vapours. ”

“ It would seem as if the appearance of the limestone was connected with the absence of the exterior line of mountains on the eastern side of the South Group. We trace it from Cass-ny-Hawin River to Purl-Keill-Moirrey (Langness-point excepted) but it never reaches to any elevation on the slope of the mountains; it is confined to the shore or its vicinity.
It lies conformably over the most superficial of the produced strata of grey-wacke, but the dip becomes less as the strata retreat farther back from the land into the sea. It may be comprehended between 10° and 20°. ”

“ 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. ”

“ I am inclined to think that several of the loose blocks I am going to describe; whenever they occur in any number, are in their birth-places.
On the beach towards Aire-point there are innumerable loose blocks of granite, one of which, rather large in its dimensions, I observed on Aire head at the elevation of 271 feet. They all in their characters differ but little the one from the other. The rock is a small-grained granite, composed of white felspar, quartz and black mica, with a few incidental plates of the latter that are white. ”

“ 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. ”

“ 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. ”

“ I met at the village of Craig-neash with a large settled block of mica-slate. The quartz is of a dirty-grey colour. The plates of mica very few and falling into decay.
††† Porphyry.
Very numerous blocks of this rock occur on the beach at Aire-point. The basis is compact felspar of a flesh-red colour, turning yellowish and earthy when going to decay. There is some hornblende disseminated throughout the mass: a few crystals of lamellar and transparent felspar are also imbedded in the basis. The rock acts strongly on the magnet. It emits when breathed upon a sensible argillaceous smell. ”

“ 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. ”

“ All the stratified rocks, but particularly those that have been deposited in a rather horizontal situation, are divided by rents perpendicular to their direction, corresponding therefore with their dip, into solids of dimensions more or less regular, and usually similar in the same individual strata. Thus if the structure of a rock be middle thick-slaty, it will divide itself spontaneously into rectangular solids. ”

“ 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. ”

“ Very little of the oldest member of the primitive class of Rocks is to be seen in the Isle of Man in Situ, nor is it the genuine old granite. Some doubts therefore may be entertained whether in the places where it occurs it does not lie in beds, rather than forming the universal foundation of the Isle. ”

“ 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. ”

“ In all the three places the vein appears adherent to the contiguous rock, whether it be greywacke or small grained granite.
At Foxdale a cross metallic vein of lead also was found running a few points from the North and South, that is to say, intersecting the main vein at a great angle; at the junction or counter, the ore grew richer, and many knockings, shods, or balls occurred. The cross course was as fertile as the main vein itself, if the information I received be correct. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ The chalk hills in the Isle of Purbeck lie in the prolongation of a line westward from the Needles: they pass through Corfe Castle, and establish a separation between two troughs, or basins. The one on the north has a gentle slope towards Poole Harbour, the other on the south is more horizontal, extending towards the sea, on the coast of which it presents a range of rather abrupt cliffs. ”