Thomas George Bonney

“ Proceeding onwards, we observe that the shore, where free from sand, consists of serpentine with intrusive veins of the newer gabbro. These become more numerous as we approach the main mass of gabbro, which is reached about a quarter of a mile from the end of the village. In plan this is an irregular oval, a little more than four miles long and two wide, which forms the elevated upland of of Crousa Down, some 300 feet above the sea. The line of the raised beach is well marked by a low cliff, and in one place by a projecting rocky plateau of considerable extent a short distance above sea-level. ”

“ There is no enstatite or diallage recognizable; but some fair-sized scales of a very dark brown and all but opaque mineral, whose cracks are filled by colourless serpentine, may have been some such mineral. With crossed prisms the larger strings vary from opaque to a pale milky grey; the smaller, commonly, have a distinct golden tinge, and the interspaces are dark or very pale dull milky grey. ”

“ About two thirds or more of the field is occupied by crystalline grains of hornblende of rather irregular outline, with very characteristic cleavage, of a green colour, and strongly dichroic. There are many small rounded or vermicular grains of a black mineral, probably magnetite, often included in the hornblende, occasional small rounded or subangular grains of clear quartz, and a few small acicular hexagonal crystals, probably apatite. The rest of the field is occupied by a kaolinized or altered felspathic mineral, which seems, in parts, as if it had never been perfectly crystallized. ”

“ Thin veins of serpentine often show in the latter; and films of the same not seldom coat joint-faces. The dyke seen in the cliff may be taken as a type of the former. On microscopic examination it proves to be a felspar basalt,—the plagioclase being, as a rule, clear, though with polarized light dull in colour, the augite little changed, and dull green spots replacing probably the olivine. There are also grains of magnetite, with brown stains from one of the other ferric oxides; and cracks are overspread by an infiltrated serpentinous mineral. ”

“ The quarry furnishes some handsome varieties of this rock, generally a compact purplish or reddish brown groundmass, mottled and veined with pistachio green, in which are small groups of bronzite crystals of greenish hue and submetallic lustre, and sometimes veins of brightish red hæmatite (no. 9) . Some masses are almost wholly of a grey-green colour; but this is only the result of decomposition; in some, minute crystals of magnetite are common. The lower part often exhibits a curious fissile structure, the cracks being filled by calcareous films. ”

“ Several small grains of picotite occur, and a good many crystals of rounded exterior and platy structure—the same which is so conspicuous to the eye by its bright metallic lustre. The rock is clearly an altered olivine rock; and on applying polarized light we see that a few grains of that mineral yet remain unchanged. The metallic mineral is all or almost all enstatite, though part of it is rather altered, and converted into a kind of serpentine ”

“ The serpentine in the cove beyond is mottled light and dark green. This rock now continues to form the cliffs above the sea for some distance. About half mile from Cadgwith (measured in a straight line) the shore is strewn with huge boulders and overhung by cliffs of a dark serpentine. ”

“ This rock also shows a banded structure.
On the eastern side of the headland we come at once on the serpentine as the predominant rock, though it is broken through by many veins of coarse gabbro. The mineral composition of these resembles that just described. Some are of considerable thickness; but others die away as mere strings. The material of the gabbro, therefore, must have been injected in a very fluid condition, and, as even the strings are coarsely crystalline, must have made its way into rock of high temperature, and have cooled down very slowly. ”

“ The latter under the microscope proves to be actinolite. The aggregated clusters of crystals are very beautiful objects with polarizer and analyzer; and the felspar crystals (which are a little decomposed) contain, especially near their edges, many small acicular crystals of the same mineral.
Close to this vein is a small dyke (6 to 12 inches wide) of a very slaggy-looking, compact, dark rock, which, I have no doubt, is an old basalt. Some of the serpentine near this weathers to a dull green colour, and contains distinct crystalline grains of magnetite. ”

“ Granules of magnetite [5] are scattered about; and there are some very irregular grains or plates of a brown hornblende, full in many parts of a black dust. This mineral appears, from its cleavage, to have a platy structure. In the coarser specimen there appears still more actinolite, and the brown hornblende grains are much larger; they appear in some cases to have been broken or partly destroyed after they had been formed. The mineral to the eye has a clove-brown colour; it much resembles anthophyllite, but is certainly not an orthorhombic mineral ”

“ I believe that this rock was originally a hornblende schist, that its entanglement with the intrusive rock affected it to some extent, and that since then it has undergone further changes, chiefly by the action of water.
Nearly above this place, at the top of the cliff (the Rill) , is a landslip, or a deserted quarry, of considerable size. Here are two granite veins in the serpentine—one about 4 feet wide, forming a curved dyke running up the face of the higher cliff, the other showing in it a short distance towards the south. Both are much decomposed and of a red colour ”

“ In the cracks of the olivine a dark ferruginous stain is deposited; then on either side of this a layer of fibrous serpentine of pale golden colour (probably chrysotile) is formed; thus the olivine grains seem traversed by an irregular network of associated dark and light strings. The interspaces then seem to be attacked; and they also are converted into serpentine ”

“ The normal rock consists now chiefly of short broad rather irregular crystals of partly altered plagioclase, with numerous microliths and aggregated small crystals of pseudomorphous actinolite. Here and there, however, portions of plagioclase crystals still remain but little altered, as well as crystals of diallage in which the change to hornblende has not been completed. Every stage of the change from plagioclase to the saussuritic mineral can be traced in various parts of the slide. There are occasional microliths and larger grains of magnetite ”

“ One mass has carried up some irregular fragments of hornblendic schist. Close by is another mass of hornblendic schist, included in the serpentine, possibly forced into it by the granite; and another included mass may be seen on the beach below. On the hillside, a short distance beyond, is a serpentine-quarry. Two of the varieties of the rock obtained here are very pretty (no. 1 [3] ) :—one, of a dull red colour, irregularly mottled with a waxy-looking dull green mineral, with occasional flakes of greenish bronzite [4] , rather hard and irregular in fracture ”

“ This rock is, in the veins, very compact, of a dull greyish green colour, extremely hard and tough, weathering brown; in general appearance it is not unlike the 18-inch dyke described above. Under the microscope it is found to be a finely crystalline rock, closely resembling the aforesaid specimen—the augitic constituent now appearing generally as a pale greyish or brownish rather fibrous mineral, very feebly dichroic, changing with polarized and analyzed light from a pale or golden yellow to a blue or puce-brown tint. The felspathic constituent also only alters from light to dark milky grey ”

“ Here and there the augite changes to pale green rather fibrous hornblende, showing marked dichroism, and rather rich colours with analyzer and polarizer. There is also a brown dichroic mineral: some of this is probably little more than a ferruginous staining; but in one case it is certainly a distinct mineral, probably an iron-mica. There is no unaltered olivine; but I think I have detected a few pseudomorphs. This rock, then, is a basalt only slightly altered. ”

“ Another specimen, broken in 1873 from a very similar dyke, shows a still greater change, the felspar crystals being still just discernible, but the actinolite more characteristic and perfectly formed.
Near the western end of the village, in the little open glen which comes down to the cove, and about 100 yards from the sea, is a quarry in serpentine. This (no. 13) is rather a dull dark variety of an aluminous aspect, and is much cracked and jointed. This is probably due to some small gabbro veins (coarsely crystalline and much decomposed) which have been exposed in opening the quarry. ”

“ The serpentine varies in character, being sometimes dull and compact, sometimes redder in colour, and containing larger bronzite crystals. The headland called the Horse is remarkable for the boldness of the jagged rocks that form its crest, which in form, colour, and even in aspect, recall memories of the gabbro of the Cuchullin Hills; the rock, however, is an ordinary serpentine.
Nearly a mile in a direct line north-west from Kynance is a cove called Gue Graze. A gully on the north side exhibits a granite vein ”

“ Here the rock is rather decomposed and traversed by many joints, coated by a white steatitic film; in parts it shows some indications of a streaky structure; the colour is a dull earthy red to brown. The rock is full of minute scales, of a talcose aspect, which gives a glimmering lustre to the broken surfaces. Fracture rather uneven (no. 7) .
The dark crags of hornblende schist which form the northern side of Pradanack headland are singularly grand, as are the cliffs of the next mass of serpentine. The junction here is difficult, to examine. ”

“ The augitic constituent is less abundant than the other two minerals. Most of it is diallage; but grains, and sometimes even parts of a crystal, seem rather to be normal augite. Occasionally the serpentine appears to have been deposited in a crack in them: but as a rule there is not the slightest advance towards a conversion of these minerals into serpentine.
The diallage occasionally assumes a dusky, finely granulated, stained aspect along the principal cleavage-planes, which I have observed in cases where a change to hornblende is commencing ”

“ On closely examining the replacing hornblende, there appears some little difference in the various aggregates—some (and these show no signs of diallage) being more "matted" in texture, if I may use the phrase, than others, and showing here and there a little of a non-doubly refracting mineral rather like serpentine. It can hardly be supposed that one part of the same mass of gabbro would be rich in olivine and another at no great distance entirely without it ”

“ It consists of quartz and felspar, both orthoclase and plagioclase. There is a scale or two of magnesia mica, a good many small clusters of minute granules of magnetite, with some fine needles and asbestiform fibres, some of which I think are tourmaline. There is an orthorhombic mineral of secondary origin here and there in the felspar, probably prehnite. One rather abundant microlith seems to be apatite; but there is probably another mineral also present. Altogether there are a good many included microliths and some minute cavities. ”

“ This rock is almost as black and compact as that of George Cove; but the surface is varied by a vein- like mottling of very dark olive-green, and by bronzite crystals about 0⋅2 inch wide with a bright submetallic lustre (no. 10) ; it weathers a dull earthy green.
Some 100 yards from this spot, along the shore, we reached an apparently faulted junction of the serpentine and hornblende schists. Directly after, the serpentine sets in again, overlying the other rock, following nearly the plane of bedding. The hornblende schist is extremely altered ”

“ Some of the iron remains as black dust; the rest is the red peroxide. The enstatite or diallage (in small crystals always) is almost replaced by an opaque brown mineral deposited between its cleavage-planes, probably some oxide of iron. Polarized light, as usual, shows the strings to be doubly refracting, the grains isotropic.
I possess four other slides, cut from specimens from the east coast, two from the Balk and two from near Poltesco ”

“ Allport, intermediate in character between that of Gue Graze and Mullion. Some unaltered olivine, a good deal of augite or diallage (it seems generally, as at Mullion, nearer the former) and perhaps a little enstatite; there are also a few semi-opaque patches which may be altered felspar as at Gue Graze. With ordinary light the serpentine is of a yellowish colour; and there is a good deal of staining with a dark ruddy-brown iron peroxide. There is a steatitic band containing enclosures of a doubly refracting variety, some of which, are more like the pseudo-organic forms of Messrs. ”

“ It has a compact, dark, dull red or purple groundmass, often mottled with a brighter red, in which are imbedded crystalline grains of a greyish white felspar, perhaps about 0⋅2 inch in diameter, and some rather smaller crystals of diallage, generally about 1/4 to 1/3 inch apart. The matrix for a quarter of an inch square or more is often unbroken by any crystals visible to the eye; occasionally, however, the felspar predominates. The rock does not vary much in texture, the smaller veins being about as coarse as the larger masses. One small boss has the groundmass a dark green instead of red. ”