Through such phases our Highlands have certainly passed. At a very early epoch the Archæan rocks of the north-west were ridged up into great mountain-masses, but before the beginning of the pre-Cambrian period wide areas of those highly-contorted rocks had already been planed across, so that when subsidence ensued the pre-Cambrian sandstones were deposited upon a gently undulating surface of highly convoluted strata. Another great epoch of mountain-making took place after Lower Silurian times, and true mountain-ranges once more appeared in the Highland area. We cannot tell how high those mountains may have been, but they might well have rivalled the Alps. After their elevation a prolonged period of erosion ensued, and the lofty mountain-land was reduced in large measure to the condition of a plain, wide areas of which were subsequently overflowed by the inland seas of Old Red Sandstone times--so that the sediments of those seas or lakes now rest with a violent unconformity on the upturned and denuded edges of the folded and contorted Silurian strata. At a later geological period the whole Highland area was elevated _en masse_, forming an undulating plateau, traversed by countless streams and rivers, some of which flowed in hollows that had existed before the beginning of Old Red Sandstone times. Since that epoch of elevation the Highland area, although subject to occasional oscillations of level, would appear to have remained more or less continuously in the condition of dry land. The result is, that the ancient plateau of erosion has been deeply incised--the denuding agents have carved it into mountain and glen--the forms and directions of which have been determined partly by the original surface-slopes of the plateau, and partly by the petrological character of the rocks and the geological structure of the ground. [Illustration: PLATE II. INFLUENCE OF ROCK STRUCTURE ON THE FORM OF THE GROUND. FIG. 1. PLATEAU OF ACCUMULATION: HORIZONTAL STRATA, DENUDED. FIG. 2. SYNCLINAL (S.O.) AND ANTICLINAL (E.) STRATA, DENUDED. FIG. 3. FAULTED STRATA, SHOWING DENUDATION. FIG. 4. MOUNTAIN OF ACCUMULATION,--VOLCANO. FIG. 5. DIAGRAMMATIC SECTION OF A TYPICAL MOUNTAIN CHAIN, OR MOUNTAINS OF UPHEAVAL. FIG. 6. TYPES OF ROCK STRUCTURE IN THE ALPS (AFTER PROF. HEIM) _The dotted lines show portions of strata, denuded._ FIG. 7. ESCARPMENTS (e) AND DIP SLOPES (d). FIG. 8. EROSION OF ANTICLINAL MOUNTAINS. FIG. 9. PLATEAU OF EROSION, SHOWING MOUNTAINS OF CIRCUMDENUDATION (aa). FIG 10. SECTION ACROSS BEN LAWERS AND LOCH TAY, SHOWING MOUNTAINS OF CIRCUMDENUDATION. The Edinburgh Geographical Institute J. G. Bartholomew F.R.G.S. ]