The whole body of the Granite of the Mount is traversed by an uninterrupted series of quartz veins, which run parallel to each other with wonderful regularity. They are very nearly vertical, and their direction is east and west. On the north-east side of the Mount many of them can be traced into the incumbent slate; a circumstance which strongly supports the idea of the cotemporaneous origin of these two rocks. In the investigation of these veins the Mineralogist may pass many an hour with satisfaction, we shall therefore point out some of the more leading phenomena which deserve his attention. De Luc observed that "that part of the vein termed in Cornwall the Capel, and on the Continent Selebanque, and which is the first stratum adherent to the sides of the fissures, changes as it passes through different kinds of strata, sometimes consisting of white Quartz, sometimes of Mica." Dr. Forbes[27] says, that "occasionally, though rarely, the line of division between the vein and the rock is tolerably distinct; frequently, however, there is rather an insensible gradation of the matter of the one into that of the other, than an obvious apposition of surfaces." The exterior parts of the veins consist of a bluish quartz, very compact, and uniformly containing a great deal of Schorl. This schorlaceous character is much more distinct towards the sides or walls of the veins, their centre being generally pure quartz; and, commonly, crystallized. In most of the veins there is a central line, or fissure, which divides them into two portions; this is formed by the close apposition and occasional union of two crystallized, or, as they may be called, drusy surfaces.
Since Veins must be considered as having once been the most active laboratories of Nature, so may they now be regarded as her most valuable cabinets of mineralogy. In those of Saint Michael's Mount may be found crystals of Apatite, from a very light to a very dark green colour, and exhibiting most of the modifications of form[28] which are common to that mineral; Oxide of Tin; Felspar; Mica beautifully crystallized in tables; Topaz in small whitish or greenish crystals,[29] both translucent and opaque, and which are extremely numerous, many hundred being observable on the face of some small blocks of granite that have fallen from the precipices.
Pinite has been said to have been also discovered in this spot. Besides which may be found that rare mineral, the Triple Sulphuret of Copper, Antimony, and Lead; Sulphuret of Tin; Malachite; Fluor Spar; and Wolfram. The occurrence of this latter mineral was, we believe, first noticed in the earlier edition of the present work, and is important in as far as its presence is generally supposed to afford decisive evidence of the primitive formation of the mountain masses in which it occurs.
This spot also presents us with several lodes of Tin and Copper; the latter may be traced for a considerable distance from the eastern to the southern base of the hill. The lode of Tin was formerly worked at the Mount, and a considerable quantity of ore obtained; any farther excavation, however, threatened to injure the foundations of the castle, and it was therefore prudently abandoned.
The remains of the Mine may be seen on the south side of the hill, and should be visited by the mineralogist, who will find in the Drift,[30] Tin crystals and Carbonate of Copper, besides some other minerals. Veins of Lead are also discoverable in the rocks. Mr. Carne[31] has lately directed the attention of the mineralogist to the veins of Mica, which have hitherto only been found in the granite of this singular spot. They are seldom more than half an inch wide; and, although tolerably straight, are very short. They generally consist of two layers of Mica in plates, which meet in the centre of the veins. Some of the masses of Granite which constitute the summit of the Mount have the appearance of an old wall retaining, in parts, a coating of plaster; this is the effect of decomposition, and of the capel having in many places remained attached to the face of the rock, after the vein itself has crumbled down.
The Botanist will also find some amusement among the rocks; he will observe the Tamarisk, (Tamarix Gallica) growing in their crevices, and relieving by a delicate verdure the harsh uniformity of their surfaces. This shrub was probably imported from Normandy by the Monks. Asplenium Marinum and Inula Helenium are also to be seen among the rocks—but let us leave the Botanist and Mineralogist to their researches, while we climb the hill and examine the venerable building on its summit.