scenic features of these beds on the theory that the basalt has thus had many lateral vents of egress, during which the unequal cooling of the associated stratified rocks has caused them to be rent from the planes of stratification, banded structure being also developed in the igneous rock.* Now, on the hypothesis so far confirmed by observation, that Salisbury Crags had not one or two vents, but many smaller ones, also parallel to their planes of jointing, and admitting a continuance of lateral push, it is easy to conceive how the steep mural precipices of the front might have been broken from their once contiguous beds, and also that great blocks might be torn from the bedding of the greenstone across and in the direction of its joints in the Hunter's Bog. This, indeed, is one way Maclaren explains the extraordinary assemblage of blocks once visible close to the great quarry, and just where the rifle range begins in the Hunter's Bog. They numbered 500, with two exceptions were all of the Salisbury Crags greenstone, and contained 5, 6, or even 8 cubic yards—the largest, indeed, must have been 12 to 16 tons. Indeed, such assemblages of blocks, not unknown in connection with modern volcanoes, may be fairly reckoned on as concomitants of crag structure. We have already seen them at Dunsappie and near the Castle rock, whilst Mr David Grieve, F.R.S.E., informs me that such an assemblage was seventy years ago a prominent feature of the valley at Dumbiedykes. Mineralogical evidences appear to indicate that the Salisbury Crags greenstone was never in a fluid form. But great heat would be developed even in such forcing and rending of the strata. Daubrée has shown that heat may be excited by rubbing dry clay three quarters of an hour, raising the mass from 18° C. to 40° C. What heat and force may then have been produced, by the longer lateral action of which we are speculating? Yet, it may be asked, do the Crags continue into Arthur Seat? Some recent speculators have pictured them doing so, forming a lava coulèe from its apex. But the soberer detailed descriptions of Maclaren favour our view, and hint that the same explanation must be extended to the crags along the Queen's Drive. In a section of the upper cliffs above the "Hause," he says there are two or three portions of detached beds, one of which is probably the upper portion of Salisbury Crag. Regarding a prominent cliff nearly opposite that opening, he says, when looked at from the south end "the position of the greenstone suggests the idea that the rock is a thin vertical mass like a vein. It may be such, but in the absence of precise data I have placed it conformably to the beds of Salisbury Crags and Samson's Ribs." Again, Mr Milne-Home, in describing the filled in lake at the gully above these latter prominences, tells that part of the angular jointing of the rocks forming the north wall of the Drive had Proceedings of the Royal Irish Academy," vol. iii. ser. ii. No. 3.

been washed into a rounded form, suggesting the removal of blocks partly by water agency.

I have already in these "Transactions" shown that in the digging of the great drain for the new infirmary evidences of the former continuity over the old town of the rocks of Salisbury Crags were disclosed. Thus, in the Pleasance, the thin bedded flagstones, caulm stones, and red earthy clays of the Cat's-Nick were found in identical order, though 150 feet or so lower in the horizon. So, too, in Rankeillor Street, but at a lower level. Last summer a water tank was made in the field behind Messrs Nelson's new printing works, just opposite the quarries on the Salisbury Crags we have been considering. In 12 feet of stiff clay at the upper end of the tank was displayed a regular zone of blocks, at equal distances, nearly all about one foot and a half in circumference, and identical in composition with the greenstone of the crags. At about 40 feet down the section the boulders stopped, homogeneous yellow clay coming on. In the drain at the north side of the factory rows of boulders similar to those of the pond were exhibited. While in digging a well within the building, at 18 feet from the surface, beds of limestone and cherty sandstone, identical with those on the opposite crags, were disclosed.

A fair interpretation of the section bears on several points of that contemporaneous series of physical changes of the neighbouring rocks we have hinted at. Firstly, the severance of Salisbury Crags appears to belong to the epoch of denudation betwixt the 400 and 200 feet contour line. Secondly, crag forming began again, after this severance, in this and all the small lochs. once covering at least the old Edinburgh area. Thirdly, the numerous trap dykes prevented this higher plane of denudation, which is 275 feet high at the works, being swept down to that 200 feet level, which as we have seen bounds the old city on all sides. Fourthly, sudden floods and torrential agencies meanwhile swept over the ground, ever and anon. The open sides of the Cowgate and Princes Street valley afforded free scope for those rapid floods, which ultimately planed down that area to the 100 feet level, partly giving its peculiar aspect to the north-west side of Salisbury Crags. Fifthly, the anticlinal arch disclosed in the Pleasance, as well as the arched character of the sandstone strata, spoken to by Professor Jamieson, along the sides of the Calton Hill, show a disposition favourable to such scooping out. These curvatures may have arisen from a further warping and twisting of the strata when Heriot Mount was sent out from its lateral vents, one of which has been minutely described by many authors. As Heriot Mount is lower than Parkside Works, moreover its dolerite is of a different composition, its place in the regular succession of interbedded trappean rocks, as first given by Maclaren, and since often repcated, must be questioned.

1

II.-On the Chemical Composition of certain Rocks from Salisbury Crags, Edinburgh. By W. IVISON MACADAM, F.C.S., F.I.C., Lecturer on Chemistry, and Analytical Chemist, Edinburgh.

(Read 18th December 1879.)

The locality from which the following minerals were obtained has long been the scene of active speculation amongst geologists. The theories advanced have increased in number year by year, and are now legion. Whilst the geologist has been busy accumulating facts as to the presence of certain minerals in the rock masses which go to compose the crags, the chemical composition and analyses seem to have been allowed to lie almost unknown. Considering that chemistry might probably aid in clearing up a few of the difficulties which come in the way of the geologist, by giving him exact data as to the composition of the masses he is working with, I have instituted a series of analyses which will be communicated to the Society in short papers.

The rocks to which this paper refers were obtained from the south half of the crags, and refer to Mr Taylor's "South Vent." Sir James Hall, in the Edinburgh Royal Society "Transactions," published a series of analyses by Dr Kennedy, detailing the results of experiments on the formation of cale spar by compression, but the greenstone spoken of in that paper was obtained from the quarry above Holyrood Palace, and at the north end of the crags.

The first of the samples analysed by me was taken from the vent itself; the second from the rock next to and surrounding the vent; whilst the third analysis shows the course of the main or unaltered rock.

No. 1 Sample. The rock or plug of the south vent is of a light gray colour, with dark green or black crystals intermixed. It is soft and friable, and is readily pulverized to a light green powder. Specific gravity 2-619. It is evidently an altered greenstone. The powder yields effervescence on treating with an acid.

No. 2 Sample, or the rock surrounding the vent, is of a much. darker colour than the rock of the vent. It is compact and hard, and shows veins and cavities filled with calcite crystals. It gives a gray powder when pulverized. Specific gravity of rock, 2.707. The powder shows effervescence on the addition of acid.

No. 3 Sample. The rock mass consists of a hard, dark-green, heavy mineral, with a specific gravity of 2.806. When reduced to powder it has a light-green colour. It is evidently greenstone. The powder effervesces slightly when treated with acid. The analyses of these three rocks gave as follows, the results being given as percentages:

These analyses are the average results of several trials which gave figures very close upon each other. The ferric oxide (FeO3) is large, and is evidently present in two forms, part being as an oxide and part as a silicate-the first soluble in acids, the second only soluble after fusion with a flux. The amount of calcic oxide in the "vent" rock, and in the rock lying next to the "vent," is considerable. In the rock composing the "vent," the calcium salt is regularly mixed throughout, but in the rock lying next to the "vent" the calcite crystals and veins are found running through the mass of the rock. This calcic oxide is evidently obtained from the dolomite vein, which forms a stratum below the greenstone. The magnesic oxide (MgO) has been derived from the same source as the lime. Cupric oxide (CuO) is present in small quantity in all of these three samples, but I have not been able as yet to obtain any surface rock from which it could have been derived other than the mass of greenstone. It is present as a soluble compound, for it is entirely contained in the acid solution of the rocks. All of the pieces of these three examples I have yet analysed contain this copper compound, and the proportions vary from a mere trace up to 0:43 per cent.

The remaining three samples with which this paper deals are from strata pierced by the "south vent."

No. 4 Sample is from a bed of limestone lying under the greenstone. It is of a reddish iron colour, and is somewhat crystalline. When acted upon by acids it effervesces much, and yields a solution with a slight yellow tinge. The specific gravity is 2.637.

No. 5 Sample was obtained from a bed of sandstone underlying the limestone. It is greenish white in colour, and is close grained. When treated with acids it shows effervescence. Specific gravity, 2.421.

No. 6 Sample is from the same stratum as No. 5, only taken further from the limestone and nearer the bottom of the bed. It has a slight green tinge of colour, and shows rust marks at parts. When acted on by acids it gives very slight effervescence. Specific gravity, 2-536.

The average results of several analysis of these three minerals yielded the following figures calculated to percentages: