Page:Popular Science Monthly Volume 39.djvu/67

Rh impresses this ignorance upon us—even as regards some of the simplest of her operations. Sometimes it is difficult to understand the conditions of original deposit. Very often it is still more difficult to understand the conditions of denudation or removal. The great earth-movements which have certainly taken place are full of mystery—the depressions and elevations, the cracks and "faults" which have dislocated the strata, the "downthrows" sometimes of thousands of feet, which have cut across the rocks as sharply as if the cutting had been effected by a knife, the overthrows and the overthrusts, the sinkings and the underthrusts, which have inverted the order of original formation, the metamorphism which has obliterated original structure here, and has left it wholly unaltered there; the vast thicknesses which are destitute of the remains of life, in juxtaposition perhaps with some one thin bed which is crowded with them; the methods by which, and the times during which, old forms of life have been destroyed, and new forms have been introduced—all these, and a thousand others, are questions on which our ignorance is profound.

Now, it is a remarkable fact that all these difficulties are, as it were, multiplied and accentuated in that very period which is nearest to us that period which was marked by the very latest changes of which geology has any cognizance—I refer to the period which is now generally called Quaternary. It is sharply marked off from previous periods by a strictly scientific definition. Shells, and particularly marine shells, may be called the time-medals of creation. Their comparative indestructibility, and the fact that the element in which their inmates live is the same element which preserves their habitations when they die, make it certain that in them Geology keeps her oldest, most complete, and most authentic record. The Quaternary period is defined as that during which innovation was stopped as regards the development of shell-life—during which no new species was born—during which we find, with a few rare exceptions, no shell which is not also an existing and a living species. As regards them, therefore, the Quaternary period is the existing period in the classifications of geology. It is the age in which we ourselves are now living. And yet this is the very period during which the greatest novelty of all seems to have been introduced, for it is in this period that we can first detect the advent of man. Moreover, it is in this period that there seem to have been some of the most mysterious earth-movements of which the science has any glimpse. Great dislocations of strata—great changes in the distribution of land and sea—great destruction of preceding forms of life, are among the familiar conceptions which its best-ascertained phenomena suggest. Nor is this all. The vanishing of preceding forms of life in many older periods