The correlation between these different loose materials and the position in which they are found helps us also to detect their origin. The loose materials bearing glacier-marks are always found resting upon surfaces which have been worn, abraded, and engraved in the same manner, while the water-worn pebbles are everywhere found resting upon rocks the abrasion of which may be traced to water. It is true that in some localities, as, for instance, in the gravel-pit of Mount Auburn, near Cambridge, large masses of glacier-worn pebbles alternate with beach-shingle; but it is easy to show that there was here a glacier advancing into the sea, crowding its front moraine and the materials carried under it over and into the shingle washed up by the waves upon the beach. Not infrequently, also, river-pebbles may be found among glacial materials. This is especially the case where, after the disappearance of large glaciers, rivers have occupied their beds. Examples of this kind may be seen in all the valleys of the Alps. But, besides the special character of the individual fragments, the true origin of any accumulation of glacier-_debris_, commonly called drift, may be detected by the total absence of stratification, so essential a feature in all water-deposits. This absence of stratification throughout its mass is, after all, the great and important characteristic of the drift; and though I have alluded to it before, I reiterate it here, as that which distinguishes it from all like accumulations under water. I may be pardoned for dwelling upon this point, because the great controversy among geologists respecting the nature and origin of the sheet of loose materials scattered over a great part of the globe turns upon it. The _debris_ of which the drift consists are thrown together pell-mell, without any arrangement according to size or weight, larger and smaller fragments being mixed so indiscriminately that the heaviest materials may be on the very summit of the mass, and the lightest at the bottom in immediate contact with the underlying rock, or the larger pieces may stand at any level in the mass of finer ones. Impalpable powder, coarse sand, rounded, polished, and scratched fragments of every size are mixed together in a homogeneous paste, in which the larger materials are imbedded, to use a homely, but expressive comparison, like raisins and currants in a pudding. The adhesive paste holding all these fragments together is, no doubt, the result of the friction to which the whole was subjected under the glacier, and which has worked some of the softer materials into a kind of cement.