Everyone knows that that superficial film of the earth’s substance,hardly ten miles thick, which is accessible to human investigation, is composedfor the most part of beds or strata of stone, the consolidated muds and sandsof former seas and lakes, which have been deposited one upon the other, andhence are the older the deeper they lie. These multitudinous strata presentsuch resemblances and differences among themselves that they are capable ofclassification into groups or formations, and these formations again arebrigaded together into still larger assemblages, called by the oldergeologists, primary, secondary, and tertiary; by the moderns, palaeozoic,mesozoic, and cainozoic: the basis of the former nomenclature being therelative age of the groups of strata; that of the latter, the kinds of livingforms contained in them.
Though but a film if compared with the total diameter of our planet, the totalseries of formations is vast indeed when measured by any human standard, and,as all action implies time, so are we compelled to regard these mineral massesas a measure of the time which has elapsed during their accumulation. Theamount of the time which they represent is, of course, in the inverseproportion of the intensity of the forces which have been in operation. If, inthe ancient world, mud and sand accumulated on sea-bottoms at tenfold theirpresent rate, it is clear that a bed of mud or sand ten feet thick would havebeen formed then in the same time as a stratum of similar materials one footthick would be formed now, and vice versa.
At the outset of his studies, therefore, the physical geologist had to choosebetween two hypotheses; either, throughout the ages which are represented bythe accumulated strata, and which we may call geologic time, the forcesof nature have operated with much the same average intensity as at present, andhence the lapse of time which they represent must be something prodigious andinconceivable, or, in the primeval epochs, the natural powers were infinitelymore intense than now, and hence the time through which they acted to producethe effects we see was comparatively short.
The earlier geologists adopted the latter view almost with one consent. Forthey had little knowledge of the present workings of nature, and they read therecords of geologic time as a child reads the history of Rome or Greece, andfancies that antiquity was grand, heroic, and unlike the present because it isunlike his little experience of the present.
Even so the earlier observers were moved with wonder at the seeming contrastbetween the ancient and the present order of nature. The elemental forcesseemed to have been grander and more energetic in primeval times. Upheaved andcontorted, rifted and fissured, pierced by dykes of molten matter or worn awayover vast areas by aqueous action, the older rocks appeared to bear witness toa state of things far different from that exhibited by the peaceful epoch onwhich the lot of man has fallen.
But by degrees thoughtful students of geology have been led to perceive thatthe earliest efforts of nature have been by no means the grandest. Alps andAndes are children of yesterday when compared with Snowdon and the Cumberlandhills; and the so-called glacial epoch—that in which perhaps the mostextensive physical changes of which any record remaining occurred—is thelast and the newest of the revolutions of the globe. And in proportion asphysical geography—which is the geology of our own epoch—has growninto a science, and the present order of nature has been ransacked to findwhat, hibernice, we may call precedents