John MacCulloch

“ I would also remark that as quartz rock is evidently recomposed from the ruins of ancient granites, it often contains all the ingredients of that substance. In the extensive remarks which I have made on this rock, as yet so little observed, I have noticed instances in which it has been mistaken for granite. In the same way it may be mistaken for gneiss, since with a stratified structure it may contain all the ingredients of that substance. Nay, even the mica may be disposed in the same parallel form in the triple compound as it is in the more ordinary mixtures of quartz and mica. ”

“ Such are all those works in architectural decoration, in which absolute whiteness and uniformity of colour, are not only unnecessary, but from their dazzling effect even injurious. The subdued tone and slight air of antiquity given to this marble by its stained and unequal colour would in these works render it of the greatest use. Its durability for the purposes of interior architecture must also be equal to that of Carrara, although there is little doubt that when exposed to the action of the weather it would like the Pentelic be liable to corrosion in those parts which abound in mica. ”

“ For two or three miles its course lies so much through alluvial matter that it offers nothing to the spectator; nor does any material feature occur before its junction with the Tarff. Occasionally it may be seen cutting its way through quartz rock, through schist, and, not long before its arrival at the Tarff; through beds of dark bluish limestone. One solitary portion of pure white limestone or marble is also visible in this part of its course. ”

“ I have been thus particular in describing this singular rock, because it has often been confounded with gneiss, from which it is strongly distinguished in its mineral character, and because it is important that a substance of so much consequence in the system of rocks as this is should have its relations accurately ascertained, and should not from laxity of definition be introduced into a series with which it has no geological connection. Many of the Western islands offer examples of it in its most genuine form. ”

“ Where the marble is at the greatest distance from the granite it differs little or nothing in hardness or composition from ordinary specimens of this substance. But wherever it approaches or comes into contact with the granite it becomes highly indurated, effervesces slowly with acids, and gives on analysis a large portion of siliceous matter. In other respects there is no obvious change at the planes of contact, and the union between the granite and limestone is so slight that the action of the water separates them. The union of the limestone with the schist is much more intimate. ”

“ We have seen that the great mass of quartz rock is followed by a small bed of limestone, and that this again is succeeded by micaceous and clay slate, terminating the series of the bedded rocks in this direction. Here then is an order of rocks different from that which we are taught to believe universal, since the micaceous schist is far removed from the granite, and only follows a succession of limestone and quartz rock. It is the limestone which immediately follows the granite, and that granite is probably a portion of the great mass which forms the central granite of Scotland. ”

“ The minuter veins of granite are generally if not always connected with the larger pieces, and they intersect and reticulated the whole limestone as they do the schist, diminishing at length to the thickness of a leaf of paper or a thread. But there is a still more remarkable arrangement of the limestone and granite. Parallel to the limestone bed or beds, and following every flexure and contortion which it undergoes with the most perfect regularity, are to be seen narrow lines rising above the general surface, and accompanying the course of the bed through its whole extent. ”

“ It is conceivable that a mass consisting of alternate layers of micaceous or argillaceous schist and limestone, a compound of which abundant examples are seen in the immediate vicinity of the rock under review, might be so acted on by heat as to admit of the conversion of its siliceous layers into the substance now found interstratified in the limestone, while this latter resisting as it is well known to do, any change from the application of heat under such circumstances, might undergo no other affection than I that softening which has led to its flexure and contortion. ”

“ Gneiss is a rare rock in Scotland, and is principally to be found in the Long island, in Tirey, Coll, Rona, Iona, and Isla; as well as on the mainland, in Glen Elg, and Morven, I do not consider the rock which is of such frequent occurrence between Blair and Loch Spey, already described in this paper, as gneiss, since although the form is laminar the structure is not foliated: nor have I observed that the gneiss y of Scotland any where alternates with mica slate, although it does with clay slate, nor that it has any immediate connection with granite, much as it resembles it in composition. ”

“ The marble itself is in most places of a pure white, a fine grain, and dry aspect, and is extremely hard. In some places it is of an ochry colour, and is interspersed by thin veins of the same substance, so as to resemble some of the palest and worst specimens of that marble known by the name of Giallo Antico. The schist which accompanies it is argillaceous and of a bluish colour, and the granite, both in the larger masses and in the ramifications, bears so near a resemblance to that formerly described that it is superfluous to describe it again. ”

“ The chief varieties of the syenitic granites are found on the slopes which descend to the Tilt, and they are almost invariably of a grey and black colour, from the predominance of the hornblende: their texture is as various as their shades of colour, the crystals of hornblende being sometimes very large and decided, and imbedded as it were in a paste of quartz and felspar, while in other extreme cases the minuteness and mixture of the several ingredients is such, that the constituents can scarcely be discerned. ”

“ On the contrary, as I have shewn that this rock belongs to the same order of rocks as mica slate, and clay slate, it will only serve to point out their concurrence in another common circumstance. It is perhaps almost superfluous to say that the transition, which I am now describing, of quartz rock into granite, is by no means a dubious one, and that it does not bear the least resemblance to the common variety above referred to, which has so often been mistaken for granite. ”

“ A more remarkable example occurs in a rock which constitutes one of the numerous beds of which Schihallien is composed. This rock is a micaceous schist, containing imbedded fragments of granite and of quartz rock, often of considerable magnitude. The larger fragments of quartz rock are sometimes partially split at right angles to its laminar structure, and these fissures are filled with the substance of the mica slate, putting on the same pseudomorphous appearance of a vein. I have found similar veins of red sandstone in the limestone of Arran, and they have also been seen in trap. ”

“ This very irregular mass of rock consists of granite and quartz rock united. The quartz rock is of a peculiar aspect, being of a bluish colour, very uniform and compact and approaching near to common quartz in character, but still shewing evident marks of foliation. It is disturbed and intersected by the granite in a way which I need, not now repeat. A thin stripe of limestone and schist may also be seen interwoven in the granite. Below the cascade there is once more a blank alluvial space of 100 yards, which is succeeded by quartz rock for about the same space ”

“ I may only say that schist, granite, and quartz rock of different aspects, with occasional masses of porphyry, succeed each other so frequently that the whole might be considered by those who are prepossessed respecting systems of alternation, as a demonstrable instance of this disposition. ”

“ Continuing to descend nothing remarkable occurs till we arrive within about two miles of Forest lodge, where a rock like those last mentioned, traverses the bed of the river. This rock consists of a great mass of red granite so mixed with quartz rock and hornblende schist, that neither pen nor pencil can describe their confusion. Limestone may be observed both at its upper and lower edge, and this is traversed and reticulated by small granite veins. The whole mass occupies a space of about 150 yards. ”

“ I have already described a quartz rock as passing by a regular transition to granite, and here the granite in the vicinity of the quartz rock is found to have a foliated tendency, a tendency which not improbably unites it at some point with the quartz rock, although I did not discover that point of perfect transition. ”

“ The most abundant is a clay slate, rarely fissile, and generally of a very compact texture and dark blackish blue colour. It sometimes possesses a glossy and unctuous surface, and passes into a sort of talcaceous schist. It is often also much penetrated with quartz, and on some occasions finely interlaminated with the same substance, very beautifully banded specimens of this variety occur near Gow's bridge. On some occasions it becomes intermixed with hornblende, which by degrees predominating, the compound passes into a hornblende schist similar to that which is so common in gneiss. ”

“ There is however little reason to doubt that the quartz forms a vein, but I suspect that the porphyry also is disposed in a similar manner, and that the appearance in question is the result of the interference of two veins. In either case the porphyry as a mineral specimen offers an aspect I believe as new as it is difficult to explain. Its basis is the usual indurated claystone, or compact felspar, or hornstone if that term be preferred, which is the most common basis of the porphyries that occur among these strata. ”

“ Where it lies in contact with quartz rock, it is commonly of a harder texture than in more distant portions, and where it is about to alternate with schist it is often interfoliated with thin lamina of micaceous or argillaceous slate. Although dark blue or grey is the predominant colour, it occasionally varies through different shades to pale grey and greenish grey, and its texture is equally liable to variations. ”

“ In a paper on Cruachan, drawn up three years ago, the same circumstance is described, and I may here repeat that the granite in question is not decomposed, and contains a very large proportion of flesh coloured felspar, the quartz and mica being in small quantities. As granite is a rare rock in Scotland, I have not been able to extend my observations on this substance far, but I have also found the same magnetic power in some of the granites which form the Ross of Mull. The porphyries also occasionally possess the same power. ”

“ Where the great limestone bed, which I have described as occupying the southern side of Glen Tilt, is about to meet the quartz rock, it forms a range of small abrupt faces or scarps extending in an interrupted manner for perhaps a mile. These may vary from ten to thirty feet in height. In wet weather small streams fall in cascades over them in two or three distinct places; and generally the surface-water from above trickles down their faces. ”

“ And since the granite of mica, felspar, and quartz, is sometimes found to contain here and there a solitary crystal of hornblende, with which in a series of gradation those varieties in which hornblende at length prevails to the exclusion of the mica are connected, it is impossible to define the point at which granite is to end and syenite is to begin: there is no difficulty in modifying the application of the adjective term. ”

“ The extent of the limestone which I have now been describing is considerable. The mass is of great thickness, and from the obliquity of its section a very large horizontal surface is exposed. As the beds run towards the north they may be traced to a considerable distance up the hill on the right of the river, but dipping to the south they plunge into the opposite hill and disappear. Independently of the steatitical matter which is mixed with this rock, some interesting minerals are found imbedded in it. ”

“ Besides mica, steatite and noble serpentine are found mixed with the white marble. The colours of these substances offer various gradations from bright yellow down to the darkest sap green. It is by these admixtures that the green and white marbles which form by far the largest portion of these beds are produced. The colours are so variously mixed, blended, and dispersed throughout the stone that numerous varieties are the result; and these are further increased by the occasional presence of dark lead blue. This mixture of serpentine with marble is by no means uncommon in Scotland. ”

“ One solitary fact well known in the potteries showing the strong affinity which exists among earths in such a state of extreme division and suspended in water, is sufficient to suggest to its the possibility of affinities still more intense existing among earths in similar circumstances, when their proportions perhaps are different, or where, in addition to a more minute division, many of them may perhaps also be in a state of solution. ”

“ These rocks, like common granite and gneiss, frequently contain epidote as a constituent incidental part, and in this also they are distinguished as well from that syenite to which I propose to limit the term, as from the greenstone of the trap family, since epidote is but very rarely found among these latter rocks. They are equally distinguished from them by the frequent presence of another mineral which, as well as the former, is peculiar to the elder rocks, Sphene. ”

“ It contains distinct crystals of felspar, but together with these, fragments of quartz are also found in it. These are most obviously fragments not crystals. They are irregular, of different sizes from that of a pea to that of an egg, and their angles are sharp. In addition to that, where the porphyry and quartz masses are in contact larger fragments are to be observed mutually connected both with the porphyry and with the quartz. Porphyry has generally been considered as a crystallized rock, yet here it offers the mixed structure of a crystallized and conglomerated one. ”

“ Those parts therefore of the granite which seem to alternate with the limestone can only be considered as portions of veins, the disposition of which, like that of trap veins in similar cases, has accidentally coincided with the direction of the limestone beds. If granite does really occur in beds, I know no reason to prevent it from alternating with limestone, but the appearance here is much too limited, and too doubtful in its origin to be admitted as an instance of such alternation. ”

“ These are succeeded by blue limestone, and subsequently by schist and granite, but in a state of disorder so inexplicable, that I do not attempt to describe their relative positions. No appearance of parallelism is to be seen in the schistose rock, but the limestone, although much bent and twisted, has the aspect either of a complicated vein, or of the edges of a bed in a vertical position, the portions of which have been split asunder and filled with other materials. ”

“ Subordination implying inferiority or dependence, it should follow that the one rock was in all cases either necessarily less in quantity, or in some way dependent on the others The term interstratified, involves neither obscurity nor hypothesis, and is amply sufficient for the purposes of description, when combined with the relative proportions and positions of the rocks in question. ↑ I may here add that I have also observed rutile in quartz in the Scarsough mountain, and accompanied by oxidulons iron in the micaceous schist of Rannoch, so that it is not a rare mineral in Scotland. ”

“ The same is true of the siliceous lamina found in the limestone, as well as of the detached pieces and small points, since they are absolutely identical in composition with the smaller reticulations.
With respect to the limestone, it is subject to a variation of aspect where it approximates to the granite which it is interesting and important to notice. The colour of the great limestone mass is lead blue of various intensity, and its texture is almost universally large grained and highly crystalline. ”