Page:A Treatise on Geology, volume 1.djvu/143

 CHAP. VI. , especially amongst the arenaceous and calcareous compounds; in other cases, particularly among the thick masses of uniformly fine grained clay slate, very obscure. Yet, in no case, have our personal investigations among the slates of Wales or Cumbria been unsuccessful in verifying the statements of Sedgwick, and detecting certain, though not obvious, proofs of consecutive depositions among all the complication introduced by later agencies.

As a general rule it may be stated that lamination prevails most in the rocks of finest grain; beds are most distinct and continuous among the coarser grauwackes; but the laminæ observed in slate rocks are not always, nor indeed frequently, the effect of intermitting subsidence of the particles from water; for, in almost all clay slates, the predominant lamination and fissility arise from a change of molecular arrangement by influences acting since the deposition of the rock. To illustrate this, let the subjoined diagrams represent portions of clay slate and grauwacke slate, alike in all respects of structure, except the nature and direction of the lamination, D being in each a plane of stratification. In grauwacke slate (No. 37.) the laminæ of deposition show on all the vertical planes, being all parallel or nearly so to the plane of stratification; in clay slate laminæ of deposition are not seen;

other laminæ, viz. those of cleavage, induced by some process since the deposition of the rock, cross the planes of stratification (seldom at right angles), so that the stone may be split by wedges almost indefinitely into thin plates, nearly in a vertical direction. In some cases, as shown in the lower part of No. 36., the laminæ of deposition remain in clay slate; and instances occur in grauwacke slates where one or more fine grained bands have