The slate formation consists of two very distinct groups, when the most characteristic rocks are alone regarded; but it is not easy to trace the boundary between them, as the contiguous rocks appear to pass gradually into each other. These groups have one character in common: viz. that they consist of several distinct kinds of rocks, each genus of which respectively may be subdivided into schistose and compact rocks; the latter are imbedded in the former—are more crystalline—and often contain clusters of their constituent minerals: so that the complicated composition of these rocks is made known by Nature’s own analysis.
The group next the granite is primary, the more remote one is transition, or as they are termed in the following pages the porphyritic and calcareous series of the slate formation. The series next the granite is characterised by its beds of porphyry, and by its abounding in veins of tin and copper ores; the other series by the frequent occurrence of calcareous spar and strata of limestone, with occasional organic remains, and by its being sparingly metalliferous, containing no tin-ore, but being productive of lead and antimony. It is within the last series that the magnesian or serpentine rocks occur, and which in the Lizard district are developed to a large extent, forming one of the most interesting geological features of Cornwall. Those who are desirous of more information concerning this slate formation, will find a copious account in the fourth volume of the Transactions of the Geological Society of Cornwall.
It is there proposed to give these rocks, whenever they possess well-marked characters, distinct names, and not to refer a great variety of rocks to the vague and indefinite genera of clay-slate and greywacké. An accurate and more extended nomenclature would have rendered the following notices much more intelligible.
In describing the general features of Cornwall, we must not omit to mention the metalliferous veins—the great source of its commercial prosperity—and the channel through which much curious information has been obtained, concerning the structure of the earth. These veins traverse indifferently both the granite and the slate, but are most abundant in the latter rock, in the vicinity of the granite. The general direction of the tin and copper veins (or lodes, as they are provincially called) is nearly E.N.E. and W.S.W., and they are crossed by another system of veins, nearly at right angles, which are not commonly metalliferous, and when they do contain ores these are often of lead, antimony, silver, and other metals. This is not however a general rule, for in the parish of St. Just, Penwith, the tin and copper ores occur in the cross veins. The course of the veins is not straight, but they are always more or less undulating, both in their direction and in their dip or underlie. Various interesting phenomena result from their meeting with and intersecting each other, known to the miner under the names of heaves, throws, and slides. This subject is replete with curious facts, but would require considerable space for their enumeration; the reader must
therefore be again referred to the work already alluded to on “Primary Geology.”
Before concluding this brief sketch, a few words must be said on the beds of clay, sand, and gravel which cover the low grounds, both on the granite and on the slate. These may be resolved into two kinds: 1st. those resulting from the decomposition of the rocks, and which are in their original position; and 2d. those which are not in situ, affording by their appearance, composition, and position, evidence of their having been transported. These deposits belong to three distinct periods, and alternate with terrestrial and marine remains, according to their situation in respect to the sea-level at their respective epochs. Each of these transports was accompanied, or followed by, a change in the sea-level: the last corresponds with that now existing; the previous one, by the elevated ancient beaches, indicates that the sea was about fifty feet above its present level; and the earliest transport, being covered with sub-marine forests, shows that the sea was at that epoch at least fifty feet lower than it now is. (See 4th vol. Geol. Trans. of Cornwall, pp. 466, et seq.) In the lowest or oldest deposit the tin-ore (stream-tin) is found in the form of sand and gravel mixed with earthy substances: it affords the purest kind of tin, known in the market by the name of grain-tin; and appears to be the source from which the ancients derived all their tin. The notices under each parish will furnish more particulars concerning these interesting deposits.
ERRATA.
VOLUME I.
- P. 16, [note,] for Glaveney, read Glaseney.
- P. 29, line [22,] for points, read parishes.
- P. 45, lines [9] and [12], for Perr, read Parr.
- P. 47, lines [7] and [8], for Pentwan, read Pentewan.
- P. 48, line [15], for Puntner, read Pentewan.
- P. 52, line [4], for Parc, read Parr.
- P. 76, line [20], for Beni, read Berri.
- P. 87, line [18], for Kint, read Kent; line [31], for has, read had.
- P. 113, line [2], for 1623, read 1643.
- P. 141, line [5 of note], for Penrith, read Penwith.
- P. 151, In Callington, line [8], for Mellior, read Mellion.
- P. 153, line [27], for sine, read sive.
- P. 158, line [15], for Bodrigar, read Bodrigan.
- P. 210, line[ 18], for Ludgnan, read Ludgvan.
- P. 222, line [7 from foot], for Solverne, read Talverne.
- P. 226, for [Mane Mine], read Manor Mine.
- P. 244, line [3 from foot], read Glanville, of Catchfrench.
- P. 260, line [15], for Disporth, read Duporth.
- P. 298, line [11 from foot], for Carreth, read Carveth.
- P. 328, line [12 from foot], for St. Veye or St. Vewe, read St. Udey or St. Kewe.
- P. 342, line [14], for Donechenir, read Donechenin.
- P. 360, line [20], read a small neat house.
- P. 397, line [19], for Seawen, read Scawen; line [25], for Treladra, read Treludra.