The effect of this long continuance of great pressure upon the pebbles of the conglomerate in certain parts of the country has been most astonishing. The granite in the pebbles still retains its characteristic crystalline structure; it has obviously not undergone anything that could be described as fusion; yet under the influence of the two factors of that pressure, namely, its intensity and its long continuance, the granite pebbles have yielded. In some cases they are slightly elongated, in others they are much elongated, while in yet others they are even rolled out flat. At different places along the valley the various phases of the transformation can be studied. We can find places where the pebbles seem little altered, and then we can trace each stage until the solid granite pebbles have, by the application of excessive pressure, been compressed into thin sheets whose character it would not have been easy to divine if it had not been possible to trace out their history. These sheets lie close and parallel, so that the material thus produced acquires some of the characteristics of slate. It splits easily along the flattened sheets, and this rolled-out conglomerate is indeed actually used as a substitute for slate, and in some places there are houses roofed with the conglomerate which has been treated in this extraordinary fashion.
This fact will illustrate a principle, already well known in the arts, that many, if not all, solids may be made to flow like liquids if only adequate pressure be applied. The making of lead tubes is a well-known practical illustration of the same principle, for these tubes are simply formed by forcing solid lead by the hydraulic press through a mould which imparts the desired form.
If then a solid can be made to behave like a liquid, even with such pressures as are within our control, how are we to suppose that the solids would behave with such pressures as those to which they are subjected in the interior of the earth? The fact is that the terms solid and liquid, at least as we understand them, appear to have no physical meaning with regard to bodies subjected to these stupendous pressures, and this must be carefully borne in mind when we are discussing the nature of the interior of the earth.