Vol. 59.] THE TLNTAGEL AND DAVIDSTOW DISTRICT. 421 



Barras Nose. — Before leaving these rocks, a brief description 

 must be given of a peculiar crag situated on the end of Barras Nose. 

 It is about 6 feet in height, consisting below of soft greenish ' slates' 

 passing upward sharply into a conspicuously- banded rock. The 

 lighter-coloured bands are composed of a granular aggregate of 

 calcite and quartz, the former predominating, with here and there 

 a flake of blue tourmaline ; the darker, of the same constituents 

 with, in addition, a few flakes of white mica and a very considerable 

 quantity of magnetite, which forms at least half the bulk of the 

 rock. These bands endure only for some 12 inches, and are suc- 

 ceeded by a dark-grey compact rock with a somewhat slaggy manner 

 of weathering, which forms eyes or lenticular bands in a matrix 

 like the lighter-coloured and softer parts of the lower crag, here, 

 as there, plentifully sprinkled with crystals of magnetite in streaks 

 or thin bands. Higher, we see the dark lenticular bands increasing, 

 the lighter-coloured bands decreasing in relative importance, until 

 the latter are observable only with difficulty. Under the microscope 

 the grey lenticular bands resemble an altered lava of a trachytic 

 type, permeated by calcite and dolomite which, in some places, 

 constitute nearly half the slide. Translucent lath-shaped felspars, 

 containing specks of carbonates and minute flakes of mica, are 

 common. The inclusions bear a relation to structural planes. 

 Dolomite or calcite is in process of replacing the felspar-laths, 

 although much of the carbonate has scarcely the appearance of 

 a pseudomorph. The slide contains much opacite and magnetite- 

 dust. These, together with a fine cryptocrystalline mosaic, form 

 the ' base ' of the rock. Coloured minerals are conspicuously 

 absent. The rock appears to be an altered lava, similar in its 

 essential characters to others from the same headland, from the right 

 bank of the Rocky Yalley, and Bossiney Haven, the rocks differing 

 merely in the relative proportions of carbonates and secondary 

 biotite, and in the size of the felspar-laths. 



The magnetite, concentrated chiefly in the lower part of the crag, 

 weathers out from the softer limestone in prominent ridges, the 

 individual crystals reaching occasionally "07 inch across. In some 

 slides they are associated with grass-green and white micas. As a 

 rule, the magnetite is idiomorphic; while interstitially, forming 

 smaller grains or as dust, is a little haematite. Nests of large dusty 

 calcite-grains, unmixed with any of quartz, constitute the section 

 apart from the magnetite-bands. A few small grains of blue 

 tourmaline occur as an accessory mineral. 1 



Crushing has somewhat obscured the relation of the carbonates 

 and the quartz, but in many instances the quartz-grains are fringed 

 by a zone of granules of the carbonate, hinting at a practically 

 simultaneous separation of the two. 



As regards the formation of the magnetite, its idiomorphic 

 outlines and habit of occurring in groups of grains militate against 



1 It is of interest to note that the tourmaline may be embedded in the 

 magnetite (rarely also a flake of green mica). These tourmalines are exceed- 

 ingly small, often as little as '0006 inch in prism-diameter. 



