The indirect geographical elements, which, as a rule, act with and intensify the direct, are mainly climatic; the prevailing winds, rainfall, mean and extreme temperatures of every locality depending on the arrangement of land and sea and of land forms. Climate thus guided affects the weathering of rocks, and so determines the kind and arrangement of soil.
It is covered with a layer of thin, dry soil, through the slow weathering of the coral rocks.
Quartz being a mineral very resistant to weathering agencies, it forms the bulk of sands and sandstones; and when the sand grains are cemented together by a later deposit of secondary quartz a rock known as quartzite results.
Mica occurs as a primary and essential constituent of igneous rocks of almost all kinds; it is also a common product of alteration of many mineral silicates, both by weathering and by contactand dynamo-metamorphic processes.
Its intensity seems due, however, in some degree at least, to the weathering of the brown fringes of the feathers which hide the more brilliant hue, and in the Atlantic islands examples are said to retain their gay tints all the year round, while throughout Europe there is scarcely a trace of them visible in autumn and winter; but, beginning to appear in spring, they reach their greatest brilliancy towards midsummer; they are never assumed by examples in confinement.
Jagged crags, sudden abysses, magnificent canyons, forests with open parks, undulating hills, mountain prairies, freaks of weathering and erosion, and the enclosing lines of the successive hog-backs afford scenery of remarkable variety and wild beauty.
It happens, however, that many rocks are not disintegrated to this extreme degree by natural processes, and weathering invariably accompanies disintegration.
The slate hills, weathering more readily, assume gentle slopes and rounded ridges, as in the high land from Holy Loch to the Kyles of Bute.
Where it is hard and jointed, weathering into large quadrangular blocks, the hills are more especially distinguished for the gnarled bossy character of their declivities, as may be seen in Ben Ledi and the heights to the north-east of it.
All streams, from the tiniest rill to the greatest river, are continually engaged in transporting downstream solid particles of rock, the product of weathering agencies in the area which they drain.