These are familiar instances of one result of what is now taking place by the crushing of the rocky masses of our cooling and descending earth's crust, every hour beneath our feet, only upon a vastly greater scale. It is in this local transformation of work into heat that I find the true origin of volcanic heat within our globe. But if we are to test this, so as in the only way possible to decide is it a true solution of this great problem, we must again ask the question, _How much?_ and to answer this, we must determine _experimentally_ how much heat can be developed by the crushing of a given volume, say a cubic mile, of such rocky materials as we know must constitute the crust of our globe down to the bottom of the known sedimentary strata, and extending to such crystalloid rocks as we may presume underlie these. We must also obtain at least approximately what are the co-efficients of _total contraction_ between fusion and atmospheric temperature of such melted rocks, basic and acid silicates, as may be deemed representative of that co-efficient for the range of volcanic fused products, basalts, trachytes, etc., which probably sufficiently nearly coincide with that of the whole non-metallic mass of our globe. The first I have determined experimentally by two different methods, but principally by the direct one of the _work_ expended in crushing prisms of sixteen representative classes of rock; the specific gravities and specific heats of which I have also determined. If H be the height of a prism of rock crushed to powder by a pressure, P, applied to two opposite faces, which, when the prism has been reduced to its volume in powder, has acted through a range of H - t, then P × (H - t) / 772