Art. IV. Remarks on the Structure of the Calton Hill, near Edinburgh, Scotland; and on the Aqueous origin of Wacke.

Art. IV. Remarks on the Structure of the Calton Hill, near Edinburgh, Scotland; and on the Aqueous origin of Wacke; by J. W. Webster, M.D. of Boston.

The country around Edinburgh is extremely interesting to the geologist, and presents numerous instances of the junction of rocks to which the advocates of the Neptunian system have referred in support of their opinion as to the aqueous origin of greenstone, basalt, and wacke; while the same examples have been cited by the Volcanists, and by those who hold an intermediate opinion. The structure of a portion of Calton hill, where the most distinct alternations of substances (whose aqueous origin none can dispute,) with pure and well characterized wacke are displayed, has not, as yet, I believe been particularly described.

Edinburgh is situated nearly in the centre of an extensive coal formation, where the usual sandstones and other coal measures are connected with the newer rocks of transition. From the coal field rise in many places beds of greenstone, in general forming small conical and round-backed hills. Other eminences are composed of amygdaloid, claystone, and other porphyries; and basalt and trap tuff occur in an overlying position. Of these, it is not my intention to speak otherwise than as conveying a general idea of the geological relation of the wacke above referred to.

The structure of Calton hill has been exposed by the recent improvements, and in particular by a section made in the construction of the new road to London. The rock occurring in greatest abundance, and which is probably the fundamental bed, is a porphyry, the basis of which in general is claystone, which in many places passes into felspar, in others becomes a distinct greenstone. Numerous veins of calcareous spar traverse it in different directions, and I am lately informed, that very beautiful examples of veins of greenstone of contemporaneous formation with the rock itself, have been discovered in the greenstone. Upon the porphyry rests a bed of trap tuff, upon this other beds of the two rocks repose, that at the summit being porphyry. The back of the hill (as we pass from the city) is a spot of peculiar interest, consisting of alternate thin beds of bituminous shale, sandstone, wacke, and clay ironstone, disposed in a manner which will be best understood by a rough outline taken on the spot.

A Porphyry.1 Bituminous shale.7 Wacke, with calc. spar.13 Wacke.
B Trap tuff.2 Wacke.8 Bituminous shale.14 Bituminous shale.
C Porphyry.3 Sandstone.9 Wacke.15 Wacke.
D Trap tuff.4 Bituminous shale, with clay ironstone.10 Bituminous shale passing on both sides into16 Bituminous shale.
E Porphyry.5 Wacke.11 Wacke—and calc. spar.17 Sandstone.
F Beds of wacke, &c. upper part concealed by vegetation.6 Bituminous shale.12 Bituminous shale.

The wacke has a greenish gray colour, which is pretty uniform. The fracture is nearly even and earthy, it is soft, yielding readily to the nail, and has a feebly shining streak. A slight stroke with the hammer causes the mass to separate in fragments of various size, the surfaces of which are often smooth and shining, each bed being composed of large distinct concretions, having a tendency to the prismatic form. This wacke fuses with difficulty before Brooke's blow-pipe. Specific gravity not determined, as it falls to pieces on being moistened.

The sandstone is for the most part gray, in some parts spotted red and brown, forming, as the section represents, the last stratum seen; the beds of sandstone are but a few inches in thickness, and the last (17) becomes less than an inch; it is probable, however, from the relative situation, from the dip and direction, that these strata are a continuation of others seen on the other side of the hill, where they are of sufficient thickness to have been quarried for the purposes of architecture. The beds of all rocks we know vary greatly in different parts, and it is not unusual for them to be some feet at one extremity, gradually decreasing till less than an inch in thickness at the other, or they may even be lost entirely, and gradually regain their former size; and it is not improbable that these beds of sandstone will be found to continue on towards the adjoining hills of Salisbury Craig and Arthur's Seat, passing under the greenstone and trap tuff.

The bituminous shale presents the usual characters; intermixed with it are numerous nodules of the common clay ironstone, the colour of which is a yellowish brown, these also frequently present characters common to the three substances, and throughout the beds, the passage from the one to the other is distinct. Whatever may be the opinions in regard to the origin of bituminous shale, there can be but one in regard to that of sandstone; and this has lately received no feeble support from the account given us by Dr. Paris, of a formation of this rock on the coast of Cornwall, where, says he, "we actually detect nature at work, and she does not refuse admittance into her manufactory, nor conceal, with her accustomed reserve, the details of the operations in which she is engaged."

From the appearances which have been thus briefly noticed, no impartial geologist, we should imagine, would infer the volcanic origin of any portion of this formation; and if the aqueous origin of sandstone can be established, that of the wacke must be the same.

From its intimate connexion with the preceding subject, Dr. Webster subjoins the following:

Extract from a Paper on a recent formation of Sandstone, occurring in various parts of the Northern coast of Cornwall; by John Ayston Paris, M.D. F.L.S., &c. &c. Published in the Transactions of the Geological Society of Cornwall, 1818.

"A very considerable portion of the northern coast of Cornwall, is covered with a calcareous sand, consisting of minute particles of comminuted shells. That part which lies between St. Ives and Padstow is more immediately the subject of the present inquiry; a tract which, with a few exceptions, is entirely covered with this species of sand; and which in some places, has accumulated in quantities so great as to have formed hills of from forty to sixty feet in elevation. A considerable area, for instance, in the parishes of Gwythian and Phillack has been thus desolated, and several churches have been inundated. In digging into these sand hills, or upon the occasional removal of some part of them by the winds, the remains of houses may be seen; and in some places, where the churchyards have been overwhelmed, a great number of human bones may be found. The sand is supposed to have been originally brought from the sea by hurricanes, probably at a remote period."——"The sand first appears in a slight but increasing state of aggregation on several parts of the shore in the bay of St. Ives; but on approaching the Gwythian river, it becomes more extensive and indurated. On the shore opposite to Godrevy Island, an immense mass of it occurs, of more than a hundred feet in length, and from twelve to twenty feet in depth, containing entire shells and fragments of clay slate; it is singular that the whole mass assumes a striking appearance of stratification. In some places it appears that attempts have been made to separate it, probably for the purpose of building; for several old houses in Gwythian are built of it."——"It is around the promontory of New Kaye that the most extensive formation of sandstone takes place. Here it may be seen in different stages of induration; from a state in which it is too friable to be detached from the rock upon which it reposes, to a hardness so considerable, that it requires a very violent blow from a sledge to break it."——"But it is on the western side of the promontory of New Kaye, in Fistril Bay, that the geologist will be most struck with the formation; for here no other rock is in sight. The cliffs, which are high, and extend for several miles, are entirely composed of it."——"The beach is covered with disjointed fragments, which have been detached from the cliff above, many of which weigh two or three tons."

There are three modes by which Dr. Paris conceives the lapidification of calcareous sand may be effected. 1st. "By the percolation of water through a hill of calcareous sand, by which it becomes impregnated with carbonate of lime." 2d. "The percolation of water through strata containing pyritical substances, by which it becomes impregnated with sulphuric salts." 3d. "The percolation of water through decomposing slate, or any ferruginous strata, by which it becomes impregnated with iron alumina, and other mineral matter."