We will take the sandstones and shales first. They are grains of sand known to mineralogists as quartz, and consisting of a substance called silica by chemists. The grains of sand are bound together by a cement which in some few cases is identical in composition with themselves, and consists of pure silica, but usually is a mixture of sandy, clayey, and other substances. The shales are made up very largely of clay, mixed, however, usually with sand and other substances, forming a conglomerate. Both sandstones and shales are divided into layers or beds, and are said to be stratified. It is this stratified or bedded structure that gives us the first clew to the way in which these rocks were formed. Rivers are constantly carrying down sand and mud into the sea or lakes, and when their flow is slackened on entering the still water the materials they bring down with them sink and are spread out in layers over the bottom. The structure of the sandstones and shales shows that they were formed in this way; they often inclose the remains of plants that have been carried down from land, and occasionally of animals that lived in the water where they were deposited.
The next we have to consider is limestone, which is mainly made up of a substance known to chemists as calcium carbonate, or carbonate of lime.
In some districts, especially in volcanic countries, springs occur very highly charged with carbonate of lime. The warm springs of Matlock are a case in point; they are probably the last vestige of volcanic action which was in operation in that neighborhood during carboniferous times. Limestone is chiefly formed by the agency of small marine creatures of low organization. By the aid of these animals the carbonate of lime is brought back to a solid form; at their death their hard parts fall to the bottom and accumulate in a mass of pure limestone, which afterward becomes solidified into limestone rock.
The information that limestone gives us is this:
When we find, as is often the case, a mass of limestone hundreds of feet thick, and composed of little else but carbonate of lime, we know that the spot where it occurs was, at the time it was formed, far out at sea, covered by the clear water of mid ocean; and when we find that this limestone grows in certain directions earthy and impure, and that layers of shale and sandstone, thin at first, but gradually thickening out in a wedge-shape form, come in between its beds, we know that in those directions we are traveling toward the shore lines of that sea whence the water was receiving from time to time supplies of muddy and sandy sediment.
The next class of rocks are the clays that are found beneath every bed of coal, and which are known as underclays, or warrant, or spavins. They vary very much in mineral composition. Sometimes they are soft clay; sometimes clay mixed with a certain portion of sand; and sometimes they contain such a large proportion of silicious matters that they become hard, flinty rock, which many of you know under the name of gannister. But all underclays agree in two points: they are all unstratified. They differ totally from the shales and sandstones in this respect, and instead of splitting up readily into thin flakes, they break up into irregular lumpy masses. And they all contain a very peculiar vegetable fossil called Stigmaria.
This strange fossil was for a long time a sore puzzle to fossil botanists, and after much discussion the question was fairly solved by Mr. Binney by the discovery of a tree embedded in the coal measures, and standing erect just as it grew, with its roots spread out into the stratum on which it stood. These roots were Stigmaria, and the stuff into which they penetrated was an underclay. Sir Charles Lyell mentions an individual sigillaria 72 feet in length found at Newcastle, and a specimen taken from the Jarrow coal mine was more than 40 feet in length and 13 feet in diameter near the base. It is not often these trees are found erect, because the action of water, combined with natural decay, has generally thrown them down. They are, however, found in very large numbers in the roof of the coal, evidently having been tossed over, and lying there flat and squeezed thin by the pressure of the measures that lie above them.
Lastly, we come to coal itself--a rock which constitutes a small portion of the whole bulk of the carboniferous deposits, but which may be fairly looked upon as the most important member of that group, both on account of its intrinsic value and also from the interest that attaches to its history. That coal is little else but mineralized vegetable matter is a point on which there has for a long time been but small doubt. The more minute investigations of recent years have not only placed this completely beyond question, but have also enabled us to say what the plants were which contributed to the formation of coal, and in some cases even to decide what portions of those plants enter into its composition. It is a thing so universally admitted on all hands, that I shall take it for granted you are all perfectly convinced that coal has been nothing in the world but a great mass of vegetable matter. The only question is: How were these great masses of vegetable matter brought together? And you must realize that they were very large masses indeed. Just to take one instance. The Yorkshire and Derbyshire coal field is somewhere about 700 to 800 square miles in area, and Lancashire about 200. Well, in both these coal fields you have a great number of beds of coal that spread over the whole of them with tolerable regularity and thickness, and very often with scarcely any break whatever. And this is only a very small portion of what must have been the original sheet of coal, so that you see we have to account for a mass of vegetable matter perfectly free from any admixture of sand, mud, or dirt, and laid down with tolerably uniform thickness over many hundreds of square miles.
At one time it was supposed that coal was formed out of dead trees and plants which were swept down by rivers into the sea, just in the same way as shales and sandstones were formed out of mud and sand so swept down. The fatal objection to this theory, however, is that rivers would not bring down dead wood alone, but they would bring down sand and mud, and other matters, and that in the bottom of the sea the dead wood would be mixed with these matters, and instead of getting a perfectly unmixed mass of vegetable matter, we should get a mixture of dead plants, sand, mud, and other things, which would give rise to something like coal, but something very different, as any one who tries to burn such coal will soon find out, from really good, pure house coal. So that this theory, which is generally known as the "drift" theory, was totally inadequate to account for the facts as we know them.
The other theory was that coal was formed out of plants and trees that grew on the spot where we now find coal itself. On this supposition it is easy to account for the absence of foreign admixtures of sand, mud, and clay in the coal; and we can also understand very much better than by the aid of the drift theory how the coal had accumulated with such wonderful uniformity of thickness over such very large areas. This theory was for some time but poorly received; but after the discovery of Sir William Logan, that every bed of coal had a bed of underclay beneath, and the discovery of Mr. Binney, that these underclays were true soils on which plants had undoubtedly grown, there was no doubt whatever that this was the real and true explanation of the matter.