Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/433

Rh As this idea had not suggested itself to me when on the rock, I did not examine, and cannot now recollect, whether it affords situations adapted for the erection of the requisite observatories, a condition without which its other conveniences would be of no avail.

The beautiful specimens of porphyry taken from this rock may render a short description of the rock itself interesting, and I shall therefore make no apology for occupying the Society a few minutes with an account of it. It is the more worthy of notice, as it is the most accessible, and the best characterized mass of porphyry which I have seen in the various parts of Scotland that I have visited. It is an island and a peninsula alternately, according to the state of the tides, and lies off the harbour of Campbelltown, which it covers from the south and east winds. It is about half a mile in length, and a quarter in breadth. From the N.W. it rises in a pretty rapid slope, and terminates to the S.E. in a precipitous face, reaching as far as the eye can guess, from one to four hundred feet. It consists of one entire mass of porphyry, which however varies much in its colour and texture in different places. It contains no other rock or vein, nor is its contact or connection with any other to be traced, but it appears to extend on all sides below the water. On a first and cursory view, it seems to be formed of upright beds, but a more accurate examination shows that this appearance is a deception, and arises from the tendency which the rock has to split in a vertical direction. Such is its outline and general aspect that it might at a distance be taken for a mass of trap, similar to those in the vicinity of Edinburgh, its leading features being the abrupt face and perpendicular fracture. On examining this great fracture more closely we