In certain dikes, and sometimes also, perhaps, in lavas known as basalts, which have flowed on the surface, the rock when cooling, from the shrinkage which then occurs, has broken in a very regular way, forming hexagonal columns which are more or less divided on their length by joints. When worn away by the agencies of decay, especially where the material forms steep cliffs, a highly artificial effect is produced, which is often compared, where cut at right angles to the columns, to pavements, or, where the division is parallel to the columns, to the pipes of an organ. What we know of dikes inclines us to the opinion that as a whole they represent movements of softened rock where the motion-compelling agent is not mainly the expansion of the contained water which gives rise to volcanic ejection, but rather in large part due to the weight of superincumbent strata setting in motion materials which were somewhat softened, and which tended to creep, as do the clays in deep coal mines. It is evident, however; it is, moreover, quite natural, that dike work is somewhat mingled with that produced by the volcanic forces; but while the line between the two actions is not sharp, the discrimination is important, and occurs with a distinctness rather unusual on the boundary line between two adjacent fields of phenomena. * * * * * We have now to consider the general effects of the earth's interior heat so far as that body of temperature tends to drive materials from the depths of the earth to the surface. This group of influences is one of the most important which operates on our sphere; as we shall shortly see, without such action the earth would in time become an unfit theatre for the development of organic life. To perceive the effect of these movements, we must first note that in the great rock-constructing realm of the seas organic life is constantly extracting from the water substances, such as lime, potash, soda, and a host of other substances necessary for the maintenance of high-grade organisms, depositing these materials in the growing strata. Into these beds, which are buried as fast as they form, goes not only these earthy materials, but a great store of the sea water as well. The result would be in course of time a complete withdrawal into the depths of the earth of those substances which play a necessary part in organic development. The earth would become more or less completely waterless on its surface, and the rocks exposed to view would be composed mainly of silica, the material which to a great extent resists solution, and therefore avoids the dissolving which overtakes most other kinds of rocks. Here comes in the machinery of the hot springs, the dikes, and the volcanoes. These agents, operating under the influence of the internal heat of the earth, are constantly engaged in bearing the earthy matter, particularly its precious more solvent parts, back to the surface.