morainic masses, which, when the ice-sheet has subsequently advanced, have met with a somewhat similar treatment, so that they represent the original contour of the till after the first ice-sheet had melted away. Others again compare drumlins with the sandbanks in rivers, and look upon them as masses of unstratified drift, slowly and locally accumulated under the irregularly moving ice-sheet, in places where more material was brought than could be carried away.

To the objection, that these hypotheses are incompatible with the erosive action generally attributed to ice, their advocates reply that this action was confined to mountain regions, where the valleys were steep, or to broken hilly tracts, where the ice must have flowed with comparative rapidity, but irregularly; while in the open lowlands and broad valleys, where it would advance with diminished but more equable motion, it would be powerless either to abrade or to erode, and thus accumulation might take place beneath it. This principle is not improbable, but if it be adopted, it will be found to create difficulties in regard to the hypothesis which attributes the larger lake basins to the excavatory action of ice; for these often lie in valleys where the slope of the ground is slight, and in districts where large masses of till have accumulated.1

1 For instance, according to Dr. Wallace (Fortnightly Review), the ice only begins to excavate at the foot of the mountains; that is to say, where the slope becomes comparatively gentle.

CHAPTER II

ICE-WORK IN GREAT BRITAIN AND IRELANDTHE DEPOSITS AND THEIR SIGNIFICANCE

OVER no small part of the lowlands in Britain, as far south as the northern margin of the Thames valley, deposits occur which are generally admitted to be indicative of a temperature considerably lower than at the present time, even if they are not the direct products of land-ice. Somewhat similar deposits reappear in a limited area on the Sussex coast. Contemporary beds very likely occur in the intervening districts; but as they nowhere exhibit characteristics distinctly glacial, all correlation can be only conjectural. Some traces of ice action probably remain in the south-west of England, but here also the evidence is less conclusive than in the central and northern regions. Among the glacial deposits of the lowlands, the most characteristic is a boulder clay. This contains numerous pieces of rock, ranging in form from rounded to angular, but as a rule more or less subangular, embedded in clay, sometimes loamy or sandy, but generally stiff and tenacious. Those of smaller size commonly are more or less worn, and are often

smoothed and striated by the action of ice; the larger, however, like the boulders in a moraine, are commonly unscratched, and little or not at all worn. The clay varies in colour and character in different parts of the country; it usually bears some resemblance to the argillaceous strata which outcrop either in the immediate neighbourhood or at some distance to the north. The smaller fragments also, as a rule, have travelled from the same quarter; but the larger boulders, especially in the more western districts, seem to occur in fairly well-defined "streams," which, however, as will be seen, do not always follow the same rule of dispersion. With this clay, from which the stones may locally disappear, masses of stratified gravel and sand are intercalated. These sometimes are mere lenticular streaks, limited in extent both horizontally and vertically; at others, they attain a thickness of several yards, and may be traced over an area many square miles in extent. They are not confined to any one horizon, but, at any rate on the lower ground, they are more abundant towards the middle part of the mass. Thus, on both the east and west sides of England, it has been found generally possible to divide the glacier deposits into an Upper and Lower Boulder Clay, separated by the so-called Middle Glacial Sands and Gravels. But in the interior and on the higher ground, one boulder clay only can be found, sometimes overlying sands and gravels, which, however, are very often absent. In other places the sands and