Now, in all this reasoning there is no hypothesis or speculation. The mason proves himself an intelligent, honest fellow, and uses his eyes and his head where many other men would perchance see very little need for the use of either. There can be no setting aside of his story; he can appeal to facts. "There," says he, "is a layer of peat with the rush-stalks and moss-fibres matted together in the soft brown mouldering substance, exactly as I have seen them a hundred times in the peat-cuttings on the moors, and I cannot but believe that they must both have had the same origin, that is, that they grew in swampy hollows of the land. There, too, lies a stratum of fresh-water shells identical with those that occur in our ponds and marshes. Although mouldering now, they are evidently not fragmentary, but entire and unbroken; some of them are young, others full-grown, and they lie grouped together as in our present lakes. Such shells could only live in fresh water, therefore the spot where I stand must have been at one time a fresh-water lake. There, again," he continues, "is a bed of oysters which cannot have been transported hither, for their valves are together, lying just as they do in our present oyster-beds. This green field, therefore, must have been at one period a muddy sea-bottom." After this manner and upon this kind of evidence must all inquiries into the past changes of the earth's surface be conducted. And provided only we proceed cautiously, reasoning from positive facts, and striving as far as possible to exhaust what Bacon calls the "negative instances," our deductions possess all the certainty of truth. For in much the same fashion do we derive no small part of our acquaintance with the early history of our own land, as well as with the arts and customs of other nations. The scattered relics turned up by the operations of the farmer--wooden canoes, flint hatchets, gold torques, bronze pots, fragments of pottery, and rusty coins--all have their bearing upon the annals of the country, and so clear is the evidence which they read out that an eminent antiquary has divided the early ages of Scotland into three periods, distinguished, from the character of their relics, as the "Stone Period," the "Bronze Period," and the "Iron Period."[57] But in such a classification the historian has little to guide him save the nature of the relics themselves. He places the rudest first, and groups the rest in succession, according to the degree of advancement in civilisation which they respectively indicate. And the grouping seems just, though in some cases objects belonging to two of these periods may have been to some extent contemporaneous, just as thatched roofs gave way to tiles, tiles to slates, and slates partly to lead, though at the present day a walk of half an hour in some localities will bring before us specimens of all these styles still in use. If, however, the relics of geological history lay