Strutcure of Formations. 61 are connedted with no feries orfuit of formations, they are affociated with the formation in which they are embedded, and it is faid that they are fubordincite to it. Roeftone in the Second Sandftone Formation, and Copper-flate in the Firft Floetz- Limeftone, are examples of this kind. Beds com- pofed of various foffils fometimes occur in different rock maffes. Thefe foffils are ufually thofe of which the rock mafs is principally compofed, irre gularly mixed, or are fimple ftones. Such beds are ufually very irregular, do not extend through the whole rock mafs, and in general exhibit apparently great irregularity in all their relations. They are" not alone capable of any difcrimination, and are re ferred to the rock in which they occur. They have been, but rather improperly, denominated foreign beds. When fingle beds, are well diftinguifbed by the kind of ftone of which they are compofed, and if their compofition (hews certain peculiarities, (as, for example, the determinate prefence of me tallic foffils, &c.), they are referred to the particu lar repojttories, which we ffiall afterwards confider particularly. Beds of Iron-pyrites and Magnetic lronftone are of this kind. Struc- i