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_.”