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Show DEGRADATION OF THE TUSHAEâ€"CONGLOMERATES. 179 scale have produced new features by uplifting the range en masse. But as these recent movements apply to the whole uplift, the relative altitudes of the loftier portion, which furnished the debris, and the less lofty portion, which has received it, have not been much, if at all, changed with respect to each other. But erosion has apparently effected what displacement has not; it has nearly equalized the levels of the two portions. The volcanic masses near the foci must have been very voluminous, for the conglomerates derived from them extend with great thickness over a large area, rivaling in bulk, if indeed they do not surpass, the enormous masses yet remaining. Wherever we find strata composed of clastic materials, the present methods of reasoning in geological science compel us to acknowledge that they have been derived from the degradation of masses of even greater magnitude.* In the case of a great sub-aerial conglomerate, formed under conditions which are still existing and a process still operating, we naturally look to the vicinity or border of the conglomerate itself for the source of the materials. We find a very obvious source to the northward. The structure of the great uplift of which the conglomerate forms a part and large masses of eruptive strata in situ, composed of materials agreeing with those found in the clastic beds, confirm this view so strongly, that there seems no room for question. But the mass of the conglomerate argues an enormous degradation. To supply so vast an accumulation the older eruptive area in the central part of the Tu.sh.ar must have been piled thousands of feet high with successive sheets no longer visible, or have been the theater of eruptions separated by long intervals of erosion, which in the long run removed the lavas as fast as they were erupted. A view which is a compromise between these two I regard as decidedly preferable, and most fully sustained by the general tenor of the evidence throughout the entire district. We may look back to a period somewhat earlier than Middle Tertiary, when the volcanic eruptions built up iEtna-like highlands of eruptive materials, not by rapidly succeeding outpours, but by alternating emission and quiescence. Between the outbreaks many years or centuries may have elapsed, but the accumulation was much more rapid for a time than * Except in cases where pulverulent and fragmentary materials have been ejected and scattered, which is not the case in the present instance. |
