In studying any extensive section of rock we find that its members may more or less readily be separated into distinct groups. Sometimes these are distinguished by what is termed unconformability, that is, the lower series has been disturbed or inclined before the upper has been deposited upon it. This is seen on a grand scale in the section Fig. 7, in the case of the Laurentian and Cambrian formations, and on a smaller scale in Fig. 8 in the unconformable superposition of Devonian conglomerate on Silurian slates at St. Abb’s Head. In the last section it is quite evident that the beds of the lower series have been bent into abrupt folds and worn away to a considerable extent before the deposition of the overlying series. In such a case we know not merely that the upper series is newer than the lower, but that some considerable time must have elapsed after the deposition of the one before the other was laid down; and we are not surprised to find that the fossils in the groups thus unconformable to each other are very different. But even when the beds are conformable, they can usually be separated into groups, depending upon differences of mineral character, or changes which have occurred in the mode of deposition. One group of beds, for example, may be largely composed of limestone, another of sandstone or shale. One group may be distinguished by containing some special mineral, as, for example, rock salt or coal, while others may be destitute of such special minerals. One group may show by its fossils that it was deposited in the sea, others may be estuarine or lacustrine. Thus we obtain the means of dividing the rocks of the earth into groups of different ages, known as “Formations,” and marking particular periods of geological time. By tracing these formations from one district or region to another, we learn the further truth that the succession is not merely local, but that, though liable to variation in detail, its larger subdivisions hold so extensively that they may be regarded as world-wide in their distribution. [Illustration: FIG. 6. Generalised section across England from Menai Straits to the Valley of the Thames.—After Ramsay. 0 Huronian? or Laurentian? 1 Cambrian and Lower Silurian. 2 Upper Silurian. 3 Devonian. 6, 7, 8 Trias and lias. 9 and 10 Jurassic. 11 Cretaceous. 12 Eocene.] [Illustration: FIG. 7.—Generalised section from the Laurentian of Canada to the coal-field of Michigan. 0 Laurentian (the Huronian is absent in the line of this section). 1 Cambrian. 2 Lower Silurian. 3 Upper Silurian. 4 Devonian. 5 Carboniferous.] [Illustration: FIG. 8.—Unconformable superposition of Devonian conglomerate on Silurian slates, at St. Abb’s Head, Berwickshire.—After Lyell.]