arc often produced subsidiary forms resembling those of granite ; and this, among other causes, has aided in supporting the erroneous belief in the stratification of that substance. In other cases, they confuse the inexperienced observer, by misleading him respecting the planes of stratification ; as happens particularly in the argillaceous schists. In the Diallage rock of Shetland, the fissures are so numerous and extensive as entirely to suppress the appearances of the sepa rating planes of the strata; so that, in determining the positions of these, recourse must be had to other indications, derived principally from the more regular adjoining rocks. When, by the repetition of beds, either of the same or of different substances, there are formed successions or series of strata, the knowledge which we obtain of the various rocks, is derived chiefly from that ob liquity of position which is fully discussed in the next chapter; in consequence of which, united to its various accidents, every member of a considerable series is some times brought into view, even within a narrow space. By a careful comparison of positions, the same object is effected, through many interruptions, and over dis tricts of great extent. It is sometimes observed that such a series preserves, for a great space, a very accurate general parallelism; the different beds being parallel among themselves. But, very frequently, the whole series is bounded, as a single bed may be, by inclined planes; in conse quence of which it gradually vanishes by extenuation. The other accidents of a single stratum are also true of a whole series; which may thus vary, by undulation or flexure, either in the line of its inclination, or in that of its direction. Thus it becomes necessary that obser vations on these lines should be regulated by a general