It is only in comparatively recent times that the important doctrine of geological continuity has come to be generally accepted, as furnishing us with a complete and satisfactory explanation of the mode of origin of the features of our globe. The great forces, which are ever at work producing modifications in those features, operate so silently and slowly, though withal so surely, that without the closest and most attentive observation their effects may be easily overlooked; while, on the other hand, there are so many phenomena upon our globe which seem at first sight to bear testimony to the action of sudden and catastrophic forces, very different to any which appear to be at present at work, that the tendency to account for all past changes by these violent actions is a very strong one. In spite of this tendency, however, the real potency of the forces now at work upon the earth's crust has gradually made its way to recognition, and the capability of these forces, when their effects are accumulated through sufficiently long periods of time, to bring about the grandest changes, is now almost universally admitted. The modern science of geology is based upon the principle that the history of the formation and development of the earth's surface-features, and of the organisms upon it, has been continuous during enormous periods of time, and that in the study of the operations taking place upon the earth at the present day, we may find the true key to the changes which have occurred during former periods. In no branch of geological science has the doctrine of continuity had to encounter so much opposition and misconception as in that which relates to the volcanic phenomena of the globe. For a long time students of rocks utterly failed to recognise any relation between the materials which have been ejected from active volcanic vents and those which have been formed by similar agencies at earlier periods of the earth's history. And what was far worse, the subject became removed from the sphere of practical scientific inquiry to that of theological controversy, those who maintained the volcanic origin of some of the older rocks being branded as the worst of heretics. [Sidenote: CONTROVERSY CONCERNING ORIGIN OF BASALT.] With the theological aspects of the great controversy concerning the origin of basalt and similar rocks--a controversy which was carried on with such violence and acrimony during the latter half of the eighteenth century--we have here nothing to do. But it may not be uninstructive to notice the causes of the strange misconceptions which for so long a period stood in the way of the acceptance of rational views upon the subject.