Society reports his examination with every possible attention to perspicuity; stating the phænomena which occur in the bed of the stream, and those on each side of it, in the form of three distinct lines or branches of his subject, and which in fact exhibit its peculiar features. He commences with the bed of the water; which, though sometimes situated in alluvial .soil, and at other times inaccessible to observation, yet displays, on various occasions, the most confused mixture of granite, schist, lime-stone, quartz, &c. in fragments and contortions which indicate some local but powerful derangement of the original structure. Through this maze of anomalies, which defies the efforts of the pen and the pencil, we cannot presume to wander: but neither can we pass altogether unnoticed the fine specimens which it affords of tremolite and sahlite, and especially of statuary marble. The author's remarks and observations on the last-mentioned substance are well worthy of transcription, and of being perused in conjunction with his excellent commentary on the same subject in his paper on the Isle of Sky, already mentioned.

The great mass of lime-stone which we shall hereafter find forming the whole of the left boundary of Glen Tilt is of a dark blue colour, with one or two exceptions which I have already described in the progress down the river. But the beds at this place are of various colours, and offer some of the most beautiful ornamental marbles which Scotland has yet produced. *

The basis of nearly the whole is a white, rather larger grained, and crystalline marble. Beds of this variety occur in a pure state, and of considerable dimensions. But as all these marbles contain more or less of mica, with which substance they are inter-stratified, the white colour is seldom pure, being mottled with the slight gray tint which mica in similar cases always produces. It cannot therefore be considered as a statuary marble, since modern artists, acquainted with the beautiful stone of Carrara, have confined their labours to this more perfect variety. It is however perfectly applicable to various architectural as well as economical objects. A marble perfectly similar to it has lately been imported from America for the same purposes, to many of which its grayish hue and low tone of colour are more applicable than the dazzling white of Carrara. It is of a larger grain and a more compact texture than the Pentelic, with which the beautiful and interesting remains imported by Lord Elgin have lately made us acquainted. But the Pentelic marble, like that of Glen Tilt, contains mica, and

*Having pointed out the circumstance to the Duke of Atholl two years ago, quarries are now opened in them, by which the numerous varieties which they contain have been more completely brought to light than they could have been by the operation of a mineralogist's hammer.'

from

from this contamination arises its fissile nature, to which we unhappily owe so much of the injury which these wonderful works have suffered. When polished, the two can scarcely be distinguished from each other; the difference in the size of their grains disappearing, and the gray and watery stains, with the brown stripes of the micaceous laminæ, equally characterizing both.

The discovery of statuary marble in the British dominions has been long a desideratum, but having already in the present volume discussed this question, I shall only briefly remark that, how much soever we may admire those wonderful sculptures by Phidias which have been executed in a marble scarcely differing in colour or quality from this of Glen Tilt, we are very well assured on examining the progress of art in Greece, that the marble of Pentelicus was only used in the deficiency of a purer and more uniform stone, and that it was abandoned when later discoveries had made the sculptors of that country acquainted with a better class of marbles. It would be a fruitless attempt to introduce the marble of Glen Tilt, or even those whiter varieties which Scotland produces, in competition with the exquisitely beautiful and easily wrought stone of Carrara, as long as these quarries remain accessible to us. For that higher class of sculpture on which the powers of genius are exerted, the proportion between the price of the wrought and unwrought article, (to use commercial phraseology,) is so unequal, that no difference in the value of the raw material can compensate for even the most trivial defects in its quality. But there remains even in the class of the fine arts a great number of uses to which the marble of Glen Tilt might with advantage be applied. 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.

Besides mica, steatite and noble serpentine are found mixed Iwith the white marble. The colours of these substances offer various gradations from bright yellow down to the darkest sapgreen. 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. On the contrary, it may be said that all the white marbles found in this country contain it in a greater or less degree. It is common in the white marble of Sky and in that of Balahulish. In that of Assynt it is more rare, but it occurs also abundantly in a white marble which I found in Glen Fernat,

not

not far from the junction of the Brerachan with the Airdle. The well known ancient green marbles owe that colour to the same admixture, which indeed in the Egyptian green, and the Verde antico, is such that the calcareous matter is overpowered by the serpentine. The aspect of the green marble of Glen Tilt is however perfectly different from that of any marbles ancient or modern which have yet been wrought, and it offers therefore a valuable addition to the arts as well as a new commodity to the list of our mineral productions. I may add that, with a similarity of composition, many specimens bear a considerable resemblance both in character and colour to some varieties of the Cipolino of the Italians. Two other distinct varieties of marble occur where the calcareous beds terminate. The one is of an uniform ochre yellow, but of a much paler tint than the Giallo antico; the other is of a flesh colour graduating into dark blue, but neither of these beds is of great magnitude."

The second line of the author's description consists of the granitic and quartzose structure of the right or northern ridge of the glen; and it is discussed with brevity, but not without interest. The confusion, observable in this department of the field of inquiry, is attributed to the discontinuous arrangement of the quartz rock, and the irregular protuberances into which the surface of the granite is formed. With respect to real relation, the quartz is superimposed on the granite. The latter frequently contains minute but well characterized crystals of sphene dispersed through it.

Of the third line, or south side of the valley, the details are likewise reviewed with due regard to dispatch; and they chiefly relate to the various aspects and bearings of the limestone, quartz, and schistose-rocks. In the quartz-rock of Ben Gloe, the contorted and conglomerate aspect of some of the beds seems to indicate that the quartz, like the schist, with which it is associated, has been in a flexible state. - In some of the rolled porphyries, found in the hills that bound the glen, Dr. Mac Culloch detected both pinite and oxyd of titanium, in a pulverulent and investing form. His own general deductions are thus recapitulated:

The course of the Tilt may therefore be considered as bounded on one side by the outer edge of the granite-mass of the Grampians, and on the other by the primary rocks which follow and are superimposed on it. These rocks consist of an alternation of limestone, schist, and quartz-rock. The bed of the river is cut upon the line of contact of these two separate classes of rock, lying upon the surface of the granite, and against the elevated edges of the stratified rocks. Its action has in various instances exposed the junction of the granite with the stratified rocks, and these exposed parts are the confused mixtures in the bed of the river which have already been described. Although the river follows

this line of junction in a general view, it does not follow it so accurately as always to keep the granite on its right bank, and the stratified rocks on its left. For this reason, the stratified rocks are sometimes seen crossing to the right bank, and even ascending high up the right side of the hill. The granite also crosses to the left in a few cases, but as it dips under the stratified rocks it is not found in the hill.-The sections which are given illustrate this variation and explain its cause. The apparent alternation of the granite with the stratified rocks is also explained by attending to this arrangement.

The granite-masses which extend beyond the general surface of the granite, and are found in the bed of the river, do not run to any extent through the mass of strata. It is therefore probable that they are not veins, but simply irregularities, of the granite.The junctions of granite in the Tilt are not therefore the transit of independent granite-veins like those of Portsoy or Rona, but the interrupted portions of a continued line of junction between a great surface of stratified rocks, and an equally extensive but irregular surface of granite. Wherever this junction is found, a complicated disturbance of the whole rocks at the point of junction is seen, and small veins of granite are observed penetrating the stratified rocks.

In these places of junction, the granite becomes so intimately mixed with the lime-stone as to alter its character; and wherever the granite is in contact both with schist and lime-stone, a similar mixture and transition between those two substances takes place. In all other cases, the schistose and calcareous rocks preserve both their regular disposition and their ordinary chemical characters.'

Subjoined to the systematic exposition of this problematical territory, are some ingenious remarks on a singular freshwater-formation of shell-marl, included in the same district; on the extensive effects of the agency of water on the rocks described; and especially on the very sensible disturbance of the magnetic needle by the granite, which forms the right boundary of the glen. This last-mentioned interesting fact suggests some important considerations for mariners, surveyors, geologists, and miners; who seem not, in general, to be fully aware of the frequency and extent of the disturbing influence of trap-rocks, porphyry, and serpentine, on the magnet.

Sketch of the Geology of the South-western Part of Somersetshire. By Leonard Horner, Esq. F.R.S. M.G.S. The tract of country, comprehended within this sketch, is that part of Somersetshire which lies on the Bristol Channel, westward of the river Parret. The leading features of its geology are formations of greywacke and conglomerate, including beds of lime-stone, and of the red rock which extends over such a considerable portion of England. Several important

points relative to the respective positions of the prevailing strata remain to be determined: but, when due allowance is made for the unfavourable circumstances under which Mr. Horner's examination was conducted,—particularly the quantity of vegetable matter which conceals large portions of the underlying rocks from view, and the great disturbance which the strata, in several places, seem to have undergone, — the results of his observations, which we have not room to specify, will not be deemed either scanty or trivial. Before he closes his report, he thus adverts to the remains of a forest now visible only at low water:

The first appearance of this submarine forest is opposite Holford, about a quarter of a mile eastward of the place where a small brook runs into the sea. From this point to the mouth of the Parret there is a flat shore, which at low water is covered with a deep and almost fluid mud. The adjoining land is protected from the sea by a high bank of pebbles, composed almost entirely of the lyas lime-stone, and which increases in height near the place where the forest begins. Here there are seen at intervals patches of various dimensions raised six or eight inches above the sand; and upon digging into these they are found to consist of a dark brown matter resembling peat or decayed vegetable substances, mixed with a plant in which the structure is entire, with twigs and small branches of wood in a soft state, and containing here and there a few nuts. This brown matter rests upon a light blue very stiff and unctuous clay, and is of various thickness; in general from a foot to eighteen inches, but in one place I observed it two feet and a half without coming to the blue clay. Trunks of trees of a very large size are found at different intervals surrounded by the brown matter, and with their roots diverging as they grow, and fixed in the blue clay. The smaller twigs and branches in the brown matter, which look like the roots of underwood, also penetrate the blue clay, and the clay contains a great deal of that particular plant which appears the least decayed in the brown matter. Besides the trunks, there are stems of great trees sunk in the brown matter and strewed about, but without any uniformity of direction: some of these I found 20 feet long, and many of them had lateral branches attached to them. In many places, but particularly nearest the blue clay, the brown matter had a strong smell resembling that of bilge water.

In order to ascertain whether the species of any of the plants contained in the brown matter could be made out, I sent specimens of it to my friend Mr. Brown of the Linnean Society, whose eminent skill in botany is so well known, requesting him to examine them. He informs me that the plant which was best preserved was the only one upon which he could pronounce with any degree of probability, the rest being too much decayed. It resembles the common Sea Grasswrack, but the leaves are so much broader that he considers it more probably the Zostera Oceanica of Linnæus it is worthy of remark, that Dr. Smith, in his Flora Bri

tannica,