Page:A Treatise on Geology, volume 1.djvu/131

 CHAP. VI.

"On the contrary, extensive and prolonged beds are very generally schistose or laminar: the strata, also, are of this character when alternating and continuous with mica slate and quartz rock."

Stratification, or bedding, independent of lamination, is less easily traceable in the gneiss and mica schist system than in most other aqueous rocks: yet sometimes in the gneiss of Strontian, the mica schist of the Trosachs, the chloritic schists of Loch Lomond, it is sufficiently plain, to be satisfactory proof of intermitting deposition of the rocks. This intermission of deposit is, perhaps, the true cause of the bedded or stratified structure in all rocks. When different sorts of matter are alternately deposited the bedding is most perfect; but the reality of aqueous deposition is often satisfactorily shown by mere variation of colour in a mass of rocks, otherwise of continuous and uniform character. Quartz rock (Balachulish) and limestone (Loch Earn, Inverary) associated with gneiss and mica schist, generally show stratification, but less perfectly than among more recent strata. A full examination of primary tracts will, probably in every instance, satisfy a candid inquirer that the gneiss and mica schist rocks are stratified; but he will certainly notice cases where the bedding of gneiss is lost, the lamination of mica schist unintelligible, and the proofs of aqueous deposition far more obscure than among later rocks. Does this prove a difference of condition in the agencies concerned in accumulating these earliest strata, or can it be explained by considering the original structures of deposition to have undergone partial or entire obliteration through the pervading influence of heat, or local proximity of igneous rocks? for both these causes are known to have produced important effects in this respect.

Superposed Structures.—So many circumstances have occurred to change the condition of rocks since their first deposition, that it is probable few or none of them now appear with their original characters of texture, structure, or position. If we represent to ourselves an extended mass of arenaceous, argillaceous, or calcareous sediment,