OCR Text |
Show 114 GEOLOGY OF TUB HIGH PLATEAUS. a general theory of volcanism, for neither the facts nor the antecedent generalizations are ready for it. Such a theory must be the work of several generations to come, and must gradually grow into form and coherence as all great theories have done heretofore. Yet there are a few conceptions of a high degree of generality which, perhaps, contain the germs of a theory, though in their present condition they are vague and formless. They may be said to resemble stones in the quarry, rough and unhewn, but which may some time become corner-stones, columns, and entablatures in the future edifice. I shall propose some of these considerations, not in the form of a connected theory of volcanism, but as partial constituents of a theory in a highly generalized form, taking care to proceed no further than existing knowledge may afford at least some justification in proceeding. I. The first consideration has reference to the probable subterranean locus of volcanic activity. In the present stage of our knowledge it seems little credible that the sources of eruptive materials can be located at very great depths. It is almost impossible that they could have emanated from a general liquid interior. Taking the common notion that the earth has formed, by cooling, an external rocky shell, enveloping a nucleus which was once an intensely heated liquid, and which may still be so, either partially or wholly, the ordinary principles of hydrostatics lead us to conclude that all the primordial volcanic energy ought to have been exhausted even before a stable crust could have been first formed. We are in the habit of regarding the earth as hot within, but gradually dissipating its heat by conduction through the crust and by radiation into space, and if this conception have any truth, or even verisimilitude, then the eruption of portions of its primordial liquid masses ought to become more and more difficult with the process of agesâ€"nay, ought to have ceased at a period long anterior to the most ancient of any of which systematic geology can take direct cognizance; for secular cooling can only strengthen the rigid envelope and continually abstract from the heated magmas below the heat which renders them liquid and eruptible. We cannot in this connection ignore the plainest consequences of hydrostatic laws. A solid crust covering a fluid nucleus, or a portion of that crust covering a large liquid vesicle, could not remain stable for an hour unless the liquid were denser |