Before entering upon a sketch of the growth of the European Continent from the earliest times until it reached its present dimensions and outlines, I will say something of the growth of continents in general, connecting these remarks with a few words of explanation respecting some geological terms, which, although in constant use, are nevertheless not clearly defined.  I will explain, at the outset, the meaning I attach to them and the sense in which I use them, that there may be no misunderstanding between me and my readers on this point.  The words Age, Epoch, Period, Formation, may be found on almost every page of any modern work on geology; but if we sift the matter carefully, we shall find that there is a great uncertainty as to the significance of these terms, and that scarcely any two geologists use them in the same sense.  Indeed, I shall not be held blameless in this respect myself; for, on looking over preceding articles, I find that I have, from old habit, used somewhat indiscriminately names which should have a perfectly definite and invariable meaning.

As long as zooelogical nomenclature was uncontrolled by any principle, the same vagueness and indecision prevailed here also.  The words Genus, Order, Class, as well as those applied to the most comprehensive division of all in the animal kingdom, the primary branches or types, were used indiscriminately, and often allowed to include under one name animals differing essentially in their structural character.  It is only since it has been found that all these groups are susceptible of limitation, according to distinct categories of structure, that our nomenclature has assumed a more precise and definite significance.  Even now there is still some inconsistency among zooelogists as to the use of special terms, arising from their individual differences in appreciating, structural features; but I believe it to be, nevertheless, true, that general orders, classes, etc., are not merely larger or smaller groups of the same kind, but are really based upon distinct categories of structure.  As soon as such a principle is admitted in geology, and investigators recognize certain physical and organic conditions, more or less general in their action, as characteristic of all those chapters in geological history designated as Ages, Epochs, Periods, Formations, etc., all vagueness will vanish from the scientific nomenclature of this department also, and there will be no hesitation as to the use of words for which we shall then have a positive, definite meaning.

Although the fivefold division of Werner, by which he separated the rocks into Primitive, Transition, Secondary, Alluvial, and Volcanic, proved to be based on a partial misapprehension of the nature of the earth-crust, yet it led to their subsequent division into the three great groups now known as the Primary, or Palaeozoic, as they are sometimes called, because here are found the first organic remains, the Secondary, and the Tertiary.