Not a few limestones consist in part of fossil corals, and owe their origin to colonies of coral polyps which built up reefs or banks of coral in the ancient seas. In the white cliffs of Dover, Flamborough Head and other places, we have a somewhat different form of calcareous rock, which in part consists of millions of minute shells of Foraminifera, in part of broken fragments of larger shells of extinct molluscs, and to some extent of the remains of siliceous sponges. As a general rule, limestones and chalk rocks are ancient sediments, formed in clear and comparatively deep water, composed in the main of carbonate of lime, in some cases with a certain amount of carbonate of magnesium, and occasionally with a considerable admixture of silica. In such rocks land-plants must necessarily be rare. There are, however, limestones which wholly or in part owe their formation to masses of calcareous algae, which grew in the form of submarine banks or on coral reefs. Occasionally the remains of these algae are clearly preserved, but frequently all signs of plant structure have been completely obliterated. Again, there occur limestone rocks formed by chemical means, and in a manner similar to that in which beds of travertine are now being accumulated. Granites, basalts, volcanic lavas, tuffs, and other igneous rocks need not claim our attention, except in such cases as permit of plant remains being found in association with these materials. Showers of ashes blown from a volcano, may fall on the surface of a lake or sea and become mixed with sand and mud of subaerial origin. Streams of lava occasionally flow into water, or they may be poured from submarine vents, and so spread out on the ocean bed with strata of sand or clay. Passing from the nature and mode of origin of the sedimentary strata to the manner of their arrangement in the Earth’s crust, we must endeavour to sketch in the merest outline the methods of stratigraphical geology. The surface of the Earth in some places stands out in the form of bare masses of rock, roughly hewn or finely carved by Nature’s tools of frost, rain or running water; in other places we have gently undulating ground with beds of rock exposed to view here and there, but for the most part covered with loose material such as gravel, sands, boulder clay and surface soil. [Sidenote: GEOLOGICAL SECTIONS.]