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60 INTERNAL STRUCTURE. diffimilar 'maffes occur in a formation, as in the cafe of Black-coal ancl Flcetz-trap, it is faid to be a compound formation. Similar rocks are often repeated in very diffe rent periods. Each ofthefe individual depofitions is a particular formation, and the whole is deno minated a feries or fiat of formations. Thus there is aLimeftone, a Porphry, a Granite Suit, &c. 38. It is a determinate character of certain for mations, to conftitute the principal mafs of the mountain in which they occur; this is the cafe with Gneifs, Clay-flate, Porphyry, and others. With other formations, on the contrary, it is as effential a charafter to occur only in fingle beds in the others, and thefe are faid to be embedded. The Older Porphyry, Limeftone, and many others, are of this kind. When fuch individual beds oc cur in different principal formations, (that is, are not confined to a fingle one), as Primitive Lime ftone and Primitive Trap in Gneifs, Mica-flate, and Clay-flate, &c.; when they, as is the cafe with thefe, form fingle independent wholes, which always con tinue the fame, notwithftanding the difference of the rocks in which they are embedded ; and laftly, when they form members of a feries of formations, as is alfo the cafe in thefe inftances; they are to be confidered as independent formations. If, on the contrary, they are confined to one Rock mafs; if they bear no marks of a whole ; and laftly, if they are
