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. Those geologists who deny the existence of this system consider these rocks as parts of the silurian system which have been most disturbed by subterranean forces, and most altered by proximity to igneous rocks. The annexed sketch (Fig. 7) will exhibit the relations here referred to. Certain portions of the silurian rocks are supposed to have been thrown into folds by the upheaval of the primary rocks. The plications nearest to the intrusive granite would be most altered. That part of the figure below the line _a a_ represents the outcropping edges as they now appear, the upper portion of the folds having been removed by some abrading cause. [Illustration: Fig. 7.] As it is yet uncertain which of these views is correct, convenience will justify us in retaining the name of Cambrian system till further investigations shall settle the question. 1. _The Silurian System._--The following tabular arrangement exhibits the divisions of the system as recognized in England, in New York, in Pennsylvania and Virginia, and in Ohio.