But though the geological systems stand thus strongly marked off from each other when viewed as a whole, their boundary lines can often be only approximately drawn, thereby reminding us that the divisions are of man's device, and can have had no place in the plans of Him who needs not to chronicle His working by years and ages, but with whom there is no past and no future. One formation insensibly passes into another just as one nation merges into those around it. There are sometimes gaps, however, between the formations, serving to mark out strongly the limits of each,[59] precisely as intervening seas and mountain-chains serve to mark put the boundaries of different peoples and tribes. [Footnote 59: Such cases, however, are probably merely local, and may have originated from some features in the ancient physical geography of the districts where they occur. For instance, it has always been thought that palæozoic ages were marked off by a strong line of demarcation from succeeding secondary times. But the gap which occurs in England, France, and Germany, is being slowly filled up from the evidence furnished by other countries, and we shall probably find in the end that the Permian dovetailed with the Trias as closely as the Silurian with the Old Red, or the Lias with the Oolite. In truth, the longer we study the past history of our planet the less do we see of hiatus and chasm and sharp clearly defined boundary line; while the doctrine of a uniform system of laws and arrangements in the physical world, first philosophically propounded in the immortal "Principles" of Sir Charles Lyell, is ever receiving fresh confirmation.] The mineral substances of which these formations consist are comparatively few in number, being chiefly varieties of sandstone, shale, conglomerate, and limestone. One sandstone can often be scarcely distinguished from another, and so also with the other rocks; hence such tests as mineralogical texture supplies can seldom be relied on to determine the age of rocks. We can prove, for example, that a series of limestones in England may be identical in age with a set of sandstones in Sweden, and with a group of shales in America, because they all contain the same or representative genera and species of organic remains. They occupy the same position in the geological scale; that is, the animals whose fossilized remains lie buried in these rocks were all living at the same time, while lime was gathering at the sea-bottom over the site of part of England, and sand was being thrown down upon a portion of what is now Sweden, and mud was accumulating over a submerged area of America. In such cases the differences of mineralogical character go for nothing in determining the age of the rocks; we have to rely solely on the embedded fossils, and on the order of superposition.