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APPENDIX.

taken away from ns), Sorby, and others, have furnished us with a bodyof evidence which reveals to us certain important physical phenomena,associated with the appearance of slaty cleavage, if they have not pro-duced it. The nature of this evidence we will now proceed to consider.

Fossil shells are found in these slate-rocks. I have here severalspecimens of such shells, occupying various positions with regard to thecleavage planes. They are squeezed, distorted, and crushed. In somecases a flattening of the convex shell occurs, in others the valves arepressed by a force which acted in the plane of their junction, but in allcases the distortion is such as leads to the inference that the rockwhich contains these shells has been subjected to enormous pressurein a direction at right angles to the planes of cleavage ; the shellsare all flattened and spread out upon these planes. I hold in myhand a fossil trilobite of normal proportions. Here is a series of fossilsof the same creature which have suffered distortion. Some havelain across, some along, and some oblique to the cleavage of the slatein which they are found; in all cases the nature of the distortionis such as required for its production a compressing force acting at rightangles to the planes of cleavage. . As the creatures lay in the mud inthe manner indicated, the jaws of a gigantic vice appear to have closedupon them and squeezed them into the shape you see. As furtherevidence of the exertion of pressure, let me introduce to your notice acase of contortion which has been adduced by Mr. Sorby. The bed-ding of the rock shown in this figure* was once horizontal; at A wehave a deep layer of mud, and at m n a layer of comparatively un-yielding gritty material; below that again, at B, we have another layerof the fine mud of which slates are formed. This mass cleaves alongthe shading lines of the diagram ; but look at the shape of the inter-mediate bed: it is contorted into a serpentine form, and leads irre-sistibly to the conclusion that the mass has been pressed together atright angles to the planes of cleavage. This action can be experi-mentally imitated, and I have here a piece of clay in which this is doneand the same result produced on a small scale. The amount of com'pression, indeed, might be roughly estimated by supposing this con'torted bed m » to be stretched out, its length measured and comparedwith the distance c d; we find in this way that the yielding of themass has been considerable.

Let me now direct your attention to another proof of pressure. Yousee the varying colours which indicate the bedding on this mass ofslate. The dark portion, as I have stated, is gritty, and composed ofcomparatively coarse particles, which, owing to their size, shape, andgravity, sink first and constitute the bottom of each layer. Graduallyfrom bottom to top the coarseness diminishes, and near the upp ersurface of each layer we have a mass of comparatively fine clean mud'

* Omitted here.