Volltext Seite (XML)
44 DESCRIPTION OF THE PRIMARY [Ch. IV. on the side of the hills in blocks three or four feet in breadth : its hornblende gradually loses its characters, until at length it cannot be recognised as such, having passed into shorl, and thus the transition is effected into proteolite, the genus last described. In like manner, proceeding over the greenstone in a con trary direction, towards the shores of Mount’s Bay, the acces sary mineral, hornblende, becomes gradually changed into actynolite, as is evident in those rocks which have a compact and crystalline structure. This genus is not of very common occurrence, and may therefore be noticed a little more in detail. The most abundant species of actynolite-rock is hard and compact, known provincially by the names of blue-stone or blue elvan. This variety is accompanied by, and gradually passes into a blue slate, which is fissile in various degrees, and may also be easily separated into small rhomboidal pieces. The massive kind occurs in elevated ridges on the coast, and is very durable, whilst the slate is decayed to a considerable depth, still retaining its form, but progressively diminishing in tenacity from the perfect rock to the surface, where it is sometimes nearly as soft as clay : the blue colour of the slate has disappeared in the decomposed rock, which is ash-white, with oclireous stains, or of an uniform pale fawn colour; this property readily distinguishes it from slaty greenstone, with which it is nearly allied. Some of the compact kinds of this genus are very beautiful, having a light violet-coloured basis, which is variegated with stripes, and marbled markings of the dark blue species. The violet colour appears to be occasioned by the intimate union of axinite with the compact felspar basis of this rock; for axinite, in crystals, is often present, in the state of patches and small veins, traversing the rock after the manner of calcareous spar in limestone. At first sight, these rocks appear to be well calculated for ornamental pur poses, but unfortunately they are traversed by numerous seams or joints, which become discoloured by exposure to the