In this outline of a large subject it has not been possible to do more than indicate very briefly the general nature of the evidence upon which the chief conclusions are based. I hope, however, to have an opportunity ere long of dealing with the whole question in detail. [Note.--Since the original publication of this Essay, renewed investigation and study have led me to conclude that the correlation of the British and Continental glacial series is even more simple than I had supposed. I believe the use of the terms "Lower" and "Upper" in connection with the "Diluvial" deposits of the Continent has hitherto blinded us to the obvious succession of the boulder-clays. In Britain we have, as shown above, a "lower boulder-clay," an "upper boulder-clay," and the still younger boulder-clays (ground-moraines), and terminal moraines of our district ice-sheets and valley-glaciers. In the low-grounds of the Continent the succession is precisely similar. Thus the lower boulder-clay that sweeps south into Saxony represents the lower boulder-clay of Britain. In like manner, the upper boulder-clay of western and middle Germany, of Poland, and western and north-western Russia, is the equivalent of our own upper boulder-clay. Lastly, the so-called "upper diluvium" and the great terminal moraines of the Baltic coast-lands are on the horizon of the younger boulder-clays and terminal moraines of the mountainous areas of the British Islands. The so-called "lower diluvium" of the Baltic coast-lands thus represents not the _lower_ but the _upper_ diluvium of western and middle Germany, Poland, etc. German geologists are of opinion that the upper boulder-clays of the Baltic coast-lands and of the valley of the Elbe are the ground-moraines of one and the same ice-sheet, which, on its retreat, piled up the terminal moraines of the Baltic Ridge. I believe the two boulder-clays in question are quite distinct, and that the terminal moraines referred to mark the furthest advance of the last great Baltic glacier. The contemporaneity of the two boulder-clays has been taken for granted simply because they are each underlaid by a lower boulder-clay. But, as we have seen, the upper boulder-clay of the Baltic coast-lands is underlaid not by one only, but by two, and in some places even by three other boulder-clays--phenomena which never present themselves in the regions not invaded by the last great Baltic glacier. Three or four boulder-clays occur in the coast-lands of the Baltic because those regions were overflowed successively by three or four separate ice-sheets. Only two boulder-clays are met with south and east of the Baltic Ridge, because the tracts lying south and south-east of that ridge were traversed by only two _mers de glace_--namely, by that of the epoch of maximum glaciation and by the less extensive ice-sheet of the next succeeding cold period. In the region between