2. Mica slate lies next above gneiss, and is a very abundant rock. As it differs from gneiss only in the proportion of mica which it contains, and as the quantity of mica in it is very different in different places, it is often difficult to make the distinction between them. It also passes by insensible degrees into the argillaceous rocks. Many of the argillaceous rocks are found, upon close examination, to contain mica in minute scales in such abundance as to make it doubtful whether they ought not to be regarded as mica slates; that is, the metamorphic action by which argillaceous slate is converted into mica slate had proceeded so far, before it was arrested, that it becomes impossible to say whether the argillaceous or micaceous characters predominate.
3. Argillaceous slate.—The last rock of this series is a slaty rock, more or less highly argillaceous. It does not differ in lithological characters from the same rock in the higher strata. It is doubtful whether the roofing-slates should be considered as belonging to the metamorphic series or not. They have been subjected to a very high degree of metamorphic action, and yet strata intimately associated with them have, in occasional instances, contained fossils.
It is not easy to fix the exact upper limit of this series. The fossils are few, obscure, and seldom met with in the lowest fossiliferous series; and the transition is very gradual from the distinctly metamorphic to the fossiliferous rocks. This renders it impossible always to determine accurately the line of separation.
The gneiss, mica slate and argillaceous slate, have the order of superposition in which they are here named. They differ only in the amount of metamorphic action to which they have been subjected; and the gneiss which is most highly metamorphic has, by being the lowest, been most acted upon,—the mica slate less, and the argillaceous slate least. In a particular locality, however, the lowest rock which was subjected to these causes of change, instead of having been of such a character as to produce gneiss, may have been a limestone, and in that case the lowest metamorphic rock would be a saccharine marble. In another locality the lowest rock may have been a sandstone, which would be converted into quartz rock. Hence there may occur, in any part of the metamorphic series, crystalline limestone, quartz rock, hornblende slate, chlorite slate, and talcose slate; and any one of these rocks may be as abundant in any particular region, as gneiss, mica slate or argillaceous slate, is in another.
The metamorphic rocks occur in all countries where there has been any considerable amount of volcanic action, and their total amount is very great; but their stratification is so confused and contorted, their superposition so irregular, and denudations have been so extensive, that no estimate can be made of their thickness. They are, perhaps, equal to all the other stratified rocks.
SECTION V.—THE FOSSILIFEROUS ROCKS.
The fossiliferous rocks are divided into seven systems, which are readily distinguished by the order of superposition, lithological characters and organic remains. These systems are the Silurian, the Old Red Sandstone, the Carboniferous, the New Red Sandstone, the Oölitic, the Cretaceous, and the Tertiary systems. There is also an eighth system now in process of formation.
It is the opinion of some geologists that there is another system situated between the metamorphic rocks and the silurian system. It has been called by Dr. Emmons, who has studied it with much care, the “Taconic System,” the Taconic Mountains, in the western part of Massachusetts, being composed of these rocks. It is the lower part of what has been called, in England and Wales, the Cambrian system.
The strata of this system have a nearly vertical position, and consist principally of black, greenish and purple slates, of great thickness. Granular quartz rock, however, occurs in considerable quantity, and in this country two thick and important beds of limestone are found. These limestones are occasionally white and crystalline. Generally, however, as a mass, they are a dark, nearly black rock, with a network of lines of a lighter color. All the clouded marbles for architectural and ornamental purposes are from these beds, and our roofing and writing slates are all obtained from the argillaceous portion of this system.
The number of species of organic remains contained in this system is very small, and these, so far as discovered, belong to the annelida, with a few doubtful cases of mollusca. This system of rocks is found coming to the surface in a large part of New England, and the eastern part of New York, also in the western part of England and Wales.