calls hemitrope; and their recent fracture sometimes exposes small specks of mica, arranged within the prisms, in a figure which represents the section of a rhomb, and to which they have, no doubt, been determined by the crystalline forces exercised at the consolidation of the mineral in which they are imbedded. The felspar is often crystallized in forms which are perfectly exhibited. Twin crystals arising from the intersection of two rhombs, and hexagonal tables, The granite bowlders on the moors are among these varieties. afford most favourable opportunities for examining such appearances; but their smooth rounded surfaces make it exceedingly difficult to detach from them illustrative specimens for the cabinet. Of the other two constituents, quartz and mica; the former is, I believe, always amorphous, and the latter generally presents the appearance of abraded fragments, rather than of regular crystals. These phenomena may be considered as the imperfect forms of a disturbed crystallization, and caused by the simultaneous consolidation of the heterogeneous ingredients.

I am aware that in mineral cabinets, specimens of Cornish granite are sometimes exhibited in which the quartz has the prismatic form with two perfect terminations. These specimens, as far as I am acquainted with them, have been procured either from the anomalous rocks of Cligga-point, or from subordinate veins; and therefore do not belong to the present part of my inquiry. The mica is occasionally crystallized in hexagonal plates; and if I have not been misinformed, a decomposing granite has also been found in the county, in which laminæ of mica have been aggregated in prismatic crystals; an appearance nearly analogous to that presented by some interesting specimens brought by Mr. Majendie from the Scilly Isles.

A variety of this rock, of comparatively small grain, and containing a great abundance of silvery mica, is so widely extended on the moors near St. Stephen's as to deserve mentioning in this place.

Nearly the same variety appears to the right of the road leading from Helston to Merazion; and from the facility with which it yields to the chisel has been extensively used in building. But the earthy state of one of its constituents makes it pervious to the moisture of the atmosphere, and therefore ill fitted for the purposes of architecture.

We may naturally expect to find the most striking phenomena, arising from the modes in which the constituents of granite have been aggregated, in the cliffs, which expose a larger denuded surface, and where the masses are, for obvious reasons, less ruinous than the blocks which lie scattered about the country. Some beautiful varieties are found between Trewavas-point and Prasands. From the eastern extremity of the latter place I obtained the following specimens *.

1. A rock resembling a fine sandstone; composed of quartz, felspar, and chlorite, with a few specks of mica.

2. A variety, larger grained, containing flakes of bronze-coloured

mica.

3. Another coarser grained; with earthy felspar of a greenish colour.

4. A beautiful light-coloured granular aggregate; porphyritic, with large and very brilliant crystals of white felspar. The large crystals sometimes prevailed nearly to the exclusion of the base: and the face of the rock then exhibited a very uncommon appearance.

In some of the irregular masses presenting the above-mentioned characters, we found apatite, crystallized in hexagonal prisms of a bluish green colour, nearly resembling the same mineral at St. Michael's Mount; and, what I consider more unusual, semitransparent schorl of a green colour.

Among the varieties of aggregation may be mentioned the strange appearance of nodular concretions, which abound in some

* See Granite series, Nos. 32—46.

part of the cliffs west of Castle-Trereen. Were it not for their situation they might be mistaken for rounded masses derived from the ruins of some other rock. A more minute examination proves them to have been contemporaneous with the rock in which they are imbedded, and composed of the same elements. They are sometimes so nearly homogeneous, that it requires an examination with a lens to make out their granular texture. But even in these instances, a fracture will often shew the crystals of felspar, passing from the surrounding granite into the imbedded nodule without any apparent interruption in their form.

It would lead me into unnecessary details to consider at greater length the varieties of granite arising from the modified aggregation of its parts. I shall therefore proceed to consider the peculiarities exhibited in the structure of the larger masses.

Wherever any natural section of the country exposes an extended surface of the granite, we find portions of it divided by fissures, which often, for a considerable extent, preserve an exact parallelism among themselves. These masses are not unfrequently subdivided by a second system of fissures, nearly perpendicular to the former; in consequence of which structure the whole aggregate becomes separated into blocks of rhomboidal form. Fine examples of this structure may be seen a few miles west of Porth-Leven; and still more striking phenomena of the same nature are exhibited in the cliffs near Lemorna-Cove and Tol-Pedn-Penwith,

It was from appearances like these, that De Luc was led to form an opinion that the granite of Cornwall was stratified. Had he indulged less in his favourite hypothesis; and made a more extended examination of the coast, he must inevitably have found that such a conclusion was untenable. If the great parallel fissures be assumed as marking the separation between strata, and as indicating their order of superposition; we shall, in almost every succeeding headland be led to the absurd conclusion, that the same

mineral mass has been deposited in beds, in one part parallel, and in another part perpendicular to the horizon; without the least indication of contortion, or subsequent catastrophe, to account for so sudden a change in their position.

The cliffs near Tol-Pedn-Penwith, which in many places make a rude approach to a columnar structure, are divided into prismatic blocks, of such regularity, both in their form and arrangement, that they convey a striking resemblance to some piece of gigantic architecture. This peculiarity of form and structure acts powerfully on our associations; and leads us to do homage to the works of nature by contrasting them with the petty operations of human skill. This separation of the mass by nearly parallel fissures, is considered as indicating the first stage of decomposition.

By the action of the atmosphere, the fissures become enlarged; the solid angles at the points of separation gradually disappear; the rhomboidal blocks approach the spheroidal form, and no longer affording a firm support to each other, the whole mass becomes ruinous, and forms an irregular heap of rounded fragments. Some part of the cliffs between Land's End and Cape Cornwall afford striking examples of this second stage of decomposition. From the upper part of the cliff being more exposed to atmospheric action than the portions immediately below them; we not unusually find these rounded masses resting on blocks which still preserve the tabular form. These spheroidal blocks will obviously rest in that position in which their lesser axes are perpendicular to the horizon; it is also clear, that if any mechanical force be applied to them they will on its removal restore themselves to the position of permanent equilibrium. Many thousand rocks which are thus circumstanced might by an adequate force be made to vibrate on their point of support. The celebrated Loggan-Stone of Castle-Trereen is above sixty tons weight, and poised on so firm a pivot, that it may be made to oscillate through a sensible arc by the mere force of the

hand*. Many examples of the forms assumed by granite during its first progress towards disintegration are to be met with among the Tors and higher parts of the interior. But by far the greater portion of these moors exhibit the rock only in its second stage of decomposition, where the soil is continually interrupted by the naked surfaces of great bowlders, which are but the nuclei of still larger blocks which once formed a continuous mass above the present surface.

Nearly all the granite of the West of England which has of late years been so extensively used in our metropolis is procured from these bowlders. The indestructible nature of one of the component parts of granite, and the structure of the whole by which it is preserved from decomposition in one direction rather than another, are circumstances favourable to the duration of buildings formed of such materials. Still, I think that stone procured from rocks which present such evident traces of decomposition ought not to be used without most suspicious examination. In addition we may remark, that many of the houses in Cornwall which have been built of granite are continually damp from the penetration of moisture through the earthy parts of the stone; and this is unfortunately the

* The separation of decomposing granite into regular prismatic blocks, has long excited the attention of Geologists. A remarkable instance of the fact is mentioned by Saussure, §. 1689. Faujas considers these forms as derived from the felspar which is the prevailing constituent. Ramond (Voyage au Mont Perdu) seems to regard the prismatic blocks as true crystalline forms derived from the combined action of all the crystalline forces exercised at the time of the solidification of the mass. He even ventures to point out the primitive form of these crystals of granite. The prismatic forms are not now considered as crystals, but as forms resulting from the original crystalline arrangement of the whole mass about different centers. From this aggregation would arise, on a great scale, a structure resembling that of the orbicular greenstone of Corsica. The aggregation, when further continued, would finally produce a more or less perfect columnar structure.

This hypothesis has been ably supported by Dr. Mac Cullock, (Geological Transactions, Vol. II. p. 66, &c.). It is an agreement with the experiments of Mr. Watt, (Phil. Trans, 1804;) and satisfactorily accounts for the principal phenomena exhibited in the decomposition of granite.