�y��vlog

“Tourmaline hornfelses” are found sometimes near the margins of tourmaline granites; they are black with small needles of schorl which under the microscope are dark brown and richly pleochroic.

Greisen is closely connected with schorl rock both in its mineralogical composition and in its mode of origin.

In the tourmaline granites prisms of black schorl occur either singly or in stellate groups.

On its northern slope is a vast mass of schorl, the celebrated Roche Rock.

Structurally, therefore, these isles are a continuation of Land's End, but the granite has become less consistent and more friable; it is largely broken into felspar, quartz, and mica, with schorl, chlorite, and hornblende.