their chisel-marks even after the lapse of two centuries. Where a soluble or easily removable matrix, however, holds the component grains together, sandstone may be rapidly disintegrated; while, if divided by well-defined laminæ, the stone is pretty sure to split up or peel off along these planes of separation, as air, rain, and frost alternately attack it.

The crystalline rocks present many interesting varieties of weathering. The joints by which they are so abundantly traversed serve as channels for the action of percolating water and frost, and hence as lines along which the rocks are split open. In such a rock as granite, for instance, where one set of joints runs in approximately parallel planes, the influence of weathering causes the rock to open into lines that closely resemble those of masonry. Every one who has climbed granite mountains will recall such groups of opened joints as are represented in Fig. 1. In many cases, the action of the weather reveals internal structures that are invisible in freshly-broken portions of the stone. Characteristic examples of this action are supplied by the onion-like crusts that peel off from the spheroidal blocks into which many diabases and basalts weather. These groups of rounded exfoliating balls are a familiar feature among the eruptive rocks of the Midland Valley (Fig. 2).

Remarkable illustrations of the unequal advance of superficial disintegration are afforded by rocks composed of materials that vary greatly in hardness within a short space. Boulder-clay, moraine-stuff, and conglomerate, for example, which are made up of blocks of rock embedded in a matrix, are liable to have their matrix much more rapidly cut away than the blocks enclosed in it, which consequently protrude from the cliffs, and sometimes form the capitals of tall pillars that are gradually cut by the rain out of the solid rock. Some excellent examples of these rain-eroded columns are to be

CHAP. II

seen in a group of ravines worn out of the Old Red Sandstone on the right bank of the Spey above Fochabers (Fig. 3). Over the whole country the decay of the surface is in progress day by day. Everywhere the superficial rocks are crumbling away, and the rain is washing their loosened

FIG. 2.-Spheroidal Weathering of Dolerite, North Queensferry. particles down the slopes into the brooks.

Of the reality

of this universal loosening and transport of material from the high grounds to the sea, impressive illustrations may be After weeks of dry weather witnessed during heavy rains.

everything looks baked and dusty. The soil crumbles into powder at a touch. Each fitful gust of wind raises a cloud of dust from the roads, and blows away the sand that has been loosened on the surface of bare rocks. But the sky darkens, and at length rain descends. In a few minutes every channel on the roadway, every gully on the slopes, every runnel and watercourse is the track of a muddy tor

FIG. 3.-Rain-eroded Pillars of Old Red Conglomerate and Boulder-clay Fochabers.

rent which sweeps down into the nearest brook. The brooks, swollen from bank to brae by the sudden descent of such innumerable tributaries, rush along laden with the fine particles of soil and disintegrated rock, which they bear into the main stream of their drainage basin. And the rivers, dark with all this accumulated mud, sweep it downward into the nearest lake or away out to sea. In a few hours,

thousands of tons of sediment may be washed off the surface of a single parish. If now we allow the multiplying power of time to tell upon this process, we can easily perceive how vast must be the result even within a comparatively brief geological period.

It is evident that apart from the varying nature of the rocks, and their rapidity or slowness in weathering, the lowering of the surface of a country by this action of air and rain cannot possibly proceed equally over the whole. Other things being equal, the rate of degradation will be regulated by the angle of declivity, being greatest where the slopes are steepest, and where, consequently, the mechanical force of descending rain is most powerful. On flat ground it must be reduced to a minimum, not only because the motion of the rain is there feeblest, but also because in many places it is over these lower tracts that the detritus swept down from higher ground is strewn.

But we have by no means exhausted all the various ways in which nature makes use of the air and meteoric influences in the sculpture of the land. Besides its action in the slow disintegration of rocks, air plays a notable part when it blows across a country. A high gale, by prostrating trees, will sometimes lay bare a whole hillside to the elements. The blowing down of woods upon low ground has so intercepted the surface-drainage that marshes have been formed, which have subsequently grown into peat-mosses.

The most familiar geological operation of wind is seen when, in dry weather, the dust is raised from roadways and fields and borne along in the air. We probably do not adequately realise the extent to which this process contributes to the removal and redistribution of disintegrated rock and soil upon the surface of the land. More obvious are the results where prevalent breezes from the sea blow across low

flat beaches of sand. Laid bare by the recession of the tide, and gradually dried on the surface, the sand is lifted up by the wind and carried landwards, where it gathers into dunes of wavy ridge and undulating hollow, which, like the crests and troughs of a billowy sea, run, in a general sense, parallel with the coast-line. Many creeks and wide bays along the coast-line of Scotland furnish illustrations of the

'Sand-built ridge

Of heaped hills that mound the sea.'

Beginning at the far north of the kingdom we find some striking dunes at the mouth of the Kyle of Durness

(Fig. 4). Another strip of blown sand skirts part of the Dornoch Firth, and a still more extensive tract on the Moray Firth, between the Nairn and the Findhorn, has a peculiar interest from the fact that it has invaded and overwhelmed large spaces of once fertile land. The old barony of Culbin has in this way been entirely obliterated. 'I