menachanite by the the Rev. Wm. Gregor (the Titane oxydé ferrifère of Haüy) is found in a stream work.
Before arriving at Port-hallo, I observed in the village of Tregollis, in the parish of St. Kevern, a bed of greenstone of small extent, and this is the last place in Cornwall where I observed that subordinate rock of the grauwacke formation. According to the geological rather than specific characters, this greenstone ought to be considered as belonging to the transition rocks.[1] The next formation in importance, which succeeds the grauwacke, is the serpentine. But although it may be seen for some miles before, in the form of large blocks scattered here and there in the fields, and forming part of the materials of the walls of the in closures, it is not upon this rock that the grauwacke immediately rests, but on a kind of talcose slate, of a greenish grey colour at the surface, tender and shining, soft to the touch, and evidently stratified: it forms a bed of small thickness, the boundaries of which it is difficult to determine. The junction may be seen in the cliffs to the S.S.E. of Port-hallo, immediately upon crossing the brook which runs through that village in its passage to the sea.
A few paces beyond this bed of talcose slate, the serpentine appears in the form of large blocks of an irregular shape, detached one from another and prospecting above the surface of the ground : these soon disappear, and on the way to St. Kevern's Church, on the S.S.W. we enter again, for a short time, upon the grauwacke, which shews that these rocks are included in an irregular manner one within the other; and in a cultivated country it is not easy to trace the line of demarcation. It is quite clear, however, that to the S.
- ↑ Secondary as well as primitive greenstone is composed of hornblende and felspar, but the grains of it are less crystalline, and less intimately blended. Brochant, Traité de Minéralogie, tome ii. p. 60.
