those of fluvio-glacial origin. Furthermore, as the terminal moraines of land-ice often rest upon other true glacial deposits, it is often difficult to know whether we are dealing with the products of one or two glaciations over limited areas. The test of superposition is often applicable, and one is enabled to obtain some clue as to the relative order of events. In England at least three periods of glaciation seem to be indicated by the glacial deposits. On the east coast the Cromer Forest Series is succeeded by the Cromer Till, and in Yorkshire the Basement Clay occupies a similar position with regard to the overlying glacial accumulations to that of the Cromer Till. Whether these deposits be marine or terrestrial, and the evidence is not yet sufficient to settle this question to the satisfaction of all geologists, there is no doubt that they are glacial. Above them, in East Anglia, lies the Contorted Drift, the origin of which is still a moot point, and it is overlain by the great Chalky Boulder Clay, which extends far and wide over East Anglia, the Midland Counties and into Yorkshire. Evidence has been adduced to connect this with the _till_ or boulder clay which spreads over the upland districts of the north of England at the foot of the main hill-systems. This set of deposits indicates a second glaciation. As the upland till is often ploughed out by glaciers which have left their traces in the form of moraines in our upland regions, we seem here to have evidence of a third glaciation, which naturally leaves no traces in the southern districts, and the exact age of this cannot be ascertained in the absence of fossil evidence, though we may provisionally refer it to the Pleistocene period. Another attempt has been made to classify the glacial deposits, on the supposition that there have been periods of elevation and depression of the land during Pleistocene times. Some writers advocate one interglacial period when the land was depressed to an extent of 1400 and perhaps 2000 feet, while others have advocated the occurrence of a number of such interglacial marine periods. The evidence for the supposed oscillations is furnished by the existence of shell-bearing sands associated with boulder clays at high levels, the best known being on Moel Tryfan in Caernarvonshire, near Macclesfield in Cheshire, and near Oswestry in Shropshire. As many geologists believe that these shells have been carried to their present position by ice in a way which it is not our province to discuss here, we may dismiss this method of classification as based upon events which cannot be proved to have occurred. In the present state of our knowledge, it is indeed best to avoid, as far as possible, classifications which are intended to be applicable over wide regions, and to devote our attention to local details, gradually piecing together the evidence which is obtained as the result of exhaustive examination of each separate area[109].