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INTRODUCTION F it were possible completely to trace the series of changes which any part of the earth's crust has undergone, geological history would everywhere embrace the whole vast range of time that has passed in the shaping of our planet to its present form. But in no place is more than a very small fragment of the record exhibited. The effects of the great cycles of earth-development have been ever varying from place to place. With the elevation of one tract into dry land, and the wearing down of its surface by the agents of sub-aerial erosion, there has gone on concurrently the depression of a neighbouring area beneath the waters, wherein the materials derived from that land were spread out in layers of sediment to become the solid strata of a newer land. Over every part of our country this chain of vicissitudes has passed unbrokenly since the remotest times to which our knowledge can reach, and it is still passing. The records of the older epochs are ground down into material for the newer history, even as the paper-maker may reduce old documents to pulp which shall in turn become a vehicle for later knowledge.

To use the well-worn but none the less faithful simile, the geological register is everywhere the mere fragment of a volume, with here and there a leaf or often only part of a leaf remaining; and it is the aim of the geologist to reconstruct the history of the past from these fragments. We might even pursue the simile further, and speak of the geology of a limited district as the fragmentary copy of a work of world-wide distribution, decipherable only by comparison and correlation with similarly imperfect copies found in other districts in constantly varying states of mutilation. From this point of view, our Surrey record is a fragment containing portions of the later chapters only, with by far the greater part of the volume missing. In other words, of the three great groups into which we divide the fossiliferous rocks, namely, Palaeozoic (computed to represent in time-value nine-tenths of the whole), Mesozoic or Secondary, and Cainozoic or Tertiary, the strata actually visible in the county (excluding the comparatively recent 'superficial' deposits) belong entirely to the later part of the Mesozoic and the earlier part of the Cainozoic. It is true that, as will be shown in the context, older rocks are known to exist at some distance below ground, but these are too deeply buried to affect the present land-surface, and our knowledge respecting them is limited to the bare fact that they have been found in certain deep borings. 1