indurated and crystalline, while the granite nas undergone a contrary change. Sir C. Lyell mentions a remarkable example of the alteration induced in stratified rocks by intrusions of syenite or granite; near Christiania, in Norway, very dark-coloured limestone is changed into white crystalline marble, and slate into mica-schist. Traces of fossils are not uncommon in some of the schistose rocks, thus unequivocally proving their metamorphic character; as, for instance, the casts of large Trilobites found by Prof. W. B. Rogers in the altered rocks near Boston, U. S. (see page 831).

In Glen Tilt, in Scotland, schist and limestone are superimposed on and traversed by granite, and the latter is intruded among the former rocks, and ramifies into innumerable veins in the most complicated manner, proving its perfect fusion when erupted.*

Granite never occurs truly stratified, but it often assumes a laminar disposition, which may be considered as a modification of concretional structure. A prismatic or cuboidal form is sometimes observable, but this appears to be the result of incipient decomposition, for the fissures become enlarged by exposure to the air and water, and the rock separates into masses resembling piles of masonry, of which the celebrated Logan or Rocking-stones, and the Cheesewring of Cornwall, are examples.†

In some instances, a tendency to a columnar arrangement is observable, as in the cliffs near the Land's End, in Cornwall. The concretionary felspathic rock of Corsica (Napoleonite or 66 Corsican granite") presents an orbicular structure, in which balls or spheroids of concentric and alternate coats of hornblende and compact felspar, are disseminated with much regularity throughout the mass.

* See the highly valuable Memoir on the Geology of Glen Tilt, by Dr. Macculloch, Geol. Trans. (first series), vol. iii. pp. 259-337.

+ See Appendix B, on the Logan-stones.

The granites of Devonshire and Cornwall are considered by Sir H. De la Beche to have been protruded after the deposition of the coal-measures of Devon, and antecedently to the Triassic series. They appear to have been thrown up through points of least resistance, in a line extending from the southern part of Devonshire to the Scilly Isles, part having protruded through the weakest places, and the remainder being concealed beneath. From the Scilly Isles to Dartmoor inclusive, there seems to have been the upthrust of one mass, which found points of less resistance amid the superincumbent accumulations, more in some places than in others. As the masses rose, the edges of the detrital, trappean, and calcareous beds against which they pressed were frequently fractured; and into these fractures the granitic matter was forced, forming veins which can often be traced terminating in fine threads; so that not only was the pressure great, but the fluidity of the igneous rock sufficient to pass into small rents and crevices.”*

A group of plutonic rocks, consisting of granite and syenite protruded through overlying schistose and carboniferous deposits, and surrounded at their base by Triassic strata, forms the range of hills known as Charnwood Forest, in Leicestershire. The highest ridge, Bardon Hill, is crested with bare and rugged masses of syenite; and in various quarries opened at the base of the hills, interesting sections are exposed of the relative positions of the crystalline masses and the sedimentary strata. This isolated cluster of hypogene rocks is within a hundred miles of the tertiary deposits of the south-east of England; and at a less distance from the metropolis than any other plutonic region.†

30. METAMORPHISM OF ROCKS.-The transition from

Memoirs of the Geological Survey of Great Britain, vol. i. p. 228, and "Geol. Observer," p. 563.

† See J. B. Jukes on the Geology of Charnwood Forest, 4to, 1842; and Excursion to Charnwood Forest, Medals of Creation, vol. ii. p. 898.

granite to porphyritic trachyte passes through infinite gradations, but all the modifications appear to be referable to the degree of incandescence of the materials, the circumstances under which they were ejected, and their slow or rapid refrigeration.* An instructive example of the passage of granite into basalt, described by Dr. Hibbert, will illustrate these remarks. In one of the Shetland Isles, a bed of basalt, extending for many miles, is seen in contact with granite. At a little distance from the junction of the rocks, the basalt contains minute particles of quartz, and these become larger and more distinct as they approach the granite: hornblende, felspar, and greenstone (the latter is a homogeneous admixture of hornblende and felspar) next appear; still nearer, the rock consists of felspar, quartz, and hornblende: and at the line of junction felspar and quartz form a mass which requires but the presence of mica to be identical with the granite in which it is insensibly lost.†

Limestone in contact with schist frequently assumes a crystalline structure, as if the same agency, which had converted the clay into schist, had extended its influence to the overlying calcareous beds. In the Isle of Man, interesting examples of this transmutation occur. In some instances the calcareous beds in contact with the fundamental rock of schist are irregular and perfectly crystalline, but change to a stratified disposition and earthy texture in proportion as they are further removed from the schist. In other places the metamorphosis takes place more gradually, each bed of limestone (Lign. 211, a, a, a) losing its stratified character, and becoming amorphous and crystalline (b) where in contact

* In M. Credner's collection, at Gotha, the Editor was kindly shown by that accomplished geologist a series of Thuringian rock-specimens, in which porphyry was seen to pass into an argillaceous schist full of granules of felspar; also a similar transition from granite to felspathic schist.

+ Edinburgh Journal of Science

with the schist (c), as is shown in this sketch; the stratified and unstratified rocks ceasing at length to possess any mineralogical distinction. And it is a remarkable and

ISLE OF MAN.

(Dr. Macculloch's Western Isles.)

a, Stratified limestone; b, Crystalline and amorphous limestone; c. Schist.

highly instructive fact, that while in the stratified limestone organic remains occur, they are altogether absent in the crystalline mass.

In the Isle of Anglesea, Carboniferous limestone and shale full of organic remains, may be traced gradually passing into hardened shale, and finally into hornstone, jasper, and analcime-rock containing garnets and copper-ore, a change due an to intrusion of greenstone-porphyry.*

In the Ural Mountains, which form the dividing range that separates the waters of Europe from those of Asia, the effect of metamorphic action is strikingly displayed. Sir Roderick Murchison emphatically remarks, that the crystalline rocks which form the axis of the anticlinal of the

Prof. Henslow; Transactions of the Philos. Soc. of Cambridge.

Ural chain are for the most part altered Silurian strata. In the short space of a mile, observes Sir Roderick, you may walk upon the edges of the partially altered beds of grit and schist, until you find them converted into amorphous quartzrock, in contact with highly crystalline greenstone; a rock which is admitted to be of igneous and intrusive character. Coralline limestone is changed into white and green marble. The intense plutonic action which effected the disturbance of the rocks of the Urals has clearly been the cause of the rich mineral productions of those regions, the metallic veins, and the mineralization and metamorphism of the sedimentary strata.*

31. PRECIOUS STONES.t-Connected with the changes to which the metamorphic rocks have been subjected, is the formation of some of those minerals which, from their beauty, splendour, and use as ornaments, are termed precious stones. The Sapphire and the Oriental Ruby, or red sapphire, which are prized next to the diamond, and almost equal that gem in hardness, are found in trap-rocks; and the common Corundum, which is a species of the same mineral, and the Emerald, occur in mica-schist. The sapphire and ruby are pure alumina crystallized; and the supposition that they have been formed by intense igneous action, is not only probable, but is rendered almost certain by the experiments of M. Gaudin, who succeeded in producing fictitious rubies, which in every respect resemble the natural gems. These were formed by submitting alumina, with a small quantity of calcined chromate of potash, to the influence of a power

* Geology of Russia, p. 357, &c.

† Mr. J. R. Jackson's work on Minerals, treating especially of their constitution, mode of occurrence, value, and uses, is full of interest and very instructive.

The sapphire affords, by analysis, 98.5 of alumina, 0·5 of lime, and 1 of oxide of iron; the ruby, 90 of alumina, 7 of silex, and 1.2 of oxide of iron. -Phillips's Mineralogy.