A little consideration will show that the first portion of this cycle of events is the one which it is most difficult to examine and study. The products of the earlier and feeble displays of volcanic activity, at any particular centre, are liable to be destroyed, or masked, during the ejection of overwhelming masses of materials in the later stages of its more matured energy. That the feeble displays of volcanic force now exhibited in some localities will gradually increase in intensity in the future, and eventually reach the grandest stage of development, there can be no reason for doubting. But, unfortunately, we are quite unable to discriminate these feeble manifestations, which are the embryonic stages in the development of grand exhibitions of the volcanic forces, from slight outbursts which die away and make no farther sign. From what has been proved concerning the true nature of volcanic action, however, it is certain that the first step towards the exhibition of such action, at any particular locality, must be the production of an aperture in the earth's crust. Only by means of such an aperture can the vapours, gases, and rocky materials reach the surface, and give rise to the phenomena there displayed. There is reason to believe that all such apertures are really of the nature of fissures, or cracks, which have been opened through the superjacent strata by the efforts of the repressed subterranean forces. Some recent writers have, it is true, endeavoured to draw a distinction between what they call 'fissure-eruptions,' and eruptions taking place from volcanic cones. But all volcanic outbursts are truly 'fissure-eruptions'--the subterranean materials finding their way to the surface through great cracks, which, in a more or less vertical position, traverse the overlying rock-masses. It is true that in many cases portions of these cracks soon get choked up, while other portions become widened, and the volcanic energy is concentrated at such spots. Thus the materials ejected from these fissures are usually emitted in greatest quantities at one or more points along the fissure, and a single great volcanic vent, or a row of smaller vents, is established upon the line at which the fissure reaches the surface. We have seen that the amount of explosive action taking place at different volcanic vents varies according to the proportion of imprisoned water contained in the lava. In the cases where there is much explosive action, vast accumulations of scoriæ, lapilli, and dust take place, and cones of great size are built up; but in those cases where the explosive action is small the lavas flow quietly from the vent, and only small scoriæ-cones are thrown up, these being probably soon swept away by the lava-currents themselves or by denuding agencies. But both kinds of eruption have equal claims to be called 'fissure-eruptions.' [Sidenote: FORMATION OF VOLCANIC FISSURES.]