II. SCOTLAND. Lewisian Gneiss.—The most ancient rocks in the British Islands, forming what is known as the Lewisian Gneiss, have now been mapped continuously throughout the whole of their extent on the mainland, from Cape Wrath to the Kyles of Skye. They have been found to occur there in two distinct conditions. Along the western borders of Sutherland and Ross they form an irregular platform on which all later formations rest. The detailed work of the survey has brought to light the fact that this platform had an exceedingly uneven surface before the very oldest of the sedimentary formations were laid down upon it. Mountains of gneiss from 2000 to 3000 feet high, with wide and deep intervening valleys, already existed before the period of the Torridon Sandstone, and were submerged beneath the waters in which that Sandstone was accumulated. But to the east of this primeval topography, owing to the gigantic dislocations which have now been traced for upwards ot 100 miles from the northern shores of Sutherland into Skye, large slices of the deeply buried gneiss have been torn off and have been driven westward upon fractured and crushed rocks of much later date. There are thus areas of gneiss which have been moved and have undergone much consequent internal rearrangement, while to the west of these the old platform, still in great part covered with the younger formations, has been left unaffected and reveals the condition of the oldest rocks at the time when the earliest of these over-lying formations was deposited upon them.
NATURE, 1877, vol. xv. p. 297, "On the Precessional Motion of a Liquid."