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 Corn-