Page:The Cornwall coast.djvu/196



GEOLOGICALLY, we are still on the mainland when we reach the isles of Scilly; they belong to the axis of the Cornish peninsula, and are in what may still be called, comparatively, the narrow seas. The hundred-fathom line lies far beyond them; these waters, though thoroughly oceanic in character, are not oceanic in depth. We may regard the islands as the last upward thrust of the granitic backbone that runs, at a diminishing gradient, from Dartmoor to Land's End, while the submarine plateau follows a similar gradient. 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. No great elevation is attained — nothing above 160 feet; the grandeur of the coasts, which certainly does not equal that of North Cornwall, consists in their rugged wildness and the fantastic weathering of their crags. Contorted formations, logans and rock-basins, reveal the decomposition of softer measures that has been proceeding for ages. The isles lie about 27 miles west of Land's End, but the journey from Penzance to St. Mary's is about