Fig. 4.—Inclined beds, holding fossil plants. Carboniferous. South Joggins, Nova Scotia.
| 1. Shale and sandstone. Plants with Spirorbis attached; rain marks (?). | |
| 2. Sandstone and shale, 8 feet. Erect Calamites. | ┐An erect coniferous (?) tree, rooted │ on the shale, passes up through 15 ┘ feet of the sandstones and shale. |
| 3. Gray sandstone, 7 feet. | |
| 4. Gray shale, 4 feet. | |
| 5. Gray sandstone, 4 feet. | |
| 6. Gray shale, 6 inches. Prostrate and erect trees, with rootlets, leaves, Naiadites, and Spirorbis on the plants. | |
| 7. Main coal-seam, 5 feet coal in two beds. | |
| 8. Underclay, with rootlets. | |
In [Fig. 4] we have a bed of coal and its accompaniments. The coal itself was produced by the slow accumulation of vegetable matter on a water-soaked soil, and this was buried under successive beds of sand and clay, now hardened into sandstone and shale, some of the beds holding trees and reed-like plants, which still stand on the soils on which they grew, and which must have been buried in sediment deposited in inundations or after subsidence of the land. In this section we may also observe that the beds are somewhat inclined; and that this is not their original position is shown by the posture of the stems of trees, once erect, but now inclined with the beds. This leads to a consideration very important with reference to our present subject; namely, that as our continents are mostly made up of beds deposited under water and afterwards elevated, these beds have in this process experienced such disturbances that they rarely retain their horizontal position, but are tilted at various angles. When we follow such inclined strata over large areas, we find that they undulate in great waves or folds, forming what are called anticlinal and synclinal lines, and that the irregularities of the surface of the land depend to a great extent on these undulations, along with the projection of hard beds whose edges protrude at the surface. In point of fact, as shown in [Fig. 5], mountain ranges depend on these crumplings of the earth’s crust; and the primary cause of these is probably the shrinkage of the mass of the earth owing to contraction in cooling. When the disturbances of beds are extreme, they often cause intricacies of structure difficult to unravel; but when of moderate extent they very much aid us in penetrating below the surface, for we can often see a great thickness of beds rising one from beneath another, and can thus know by mere superficial examination the structure of the earth to a great depth. It thus happens that geologists reckon the thickness of the stratified deposits of the crust of the earth at more than 70,000 feet, though they cannot penetrate it perpendicularly to more than a fraction of that depth. The two sections, [Figs. 6 and 7], showing the sequence of beds in England and in the northern part of North America, will serve, if studied by the reader, to show how, by merely travelling over the surface and measuring the upturned edges of beds, many thousands of feet of deposits may be observed, and their relative ages distinctly ascertained.
Fig. 5.—Ideal section of the Apalachian Mountains showing folding of the earth’s crust.
a, Anticlinal axes. b, Overturned strata. c, Synclinals. d, Unconformable beds.
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.