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Show AUTHOR'S PREFACE. xix the old centers, however, is by no means impossible. In a considerable number of cases the larger and more important centers are still discernible, though some are doubtful and exceedingly indistinct. The 6bscurity probably arises in many cases from the fact that while the greater accumulations of lavas outflowed from great central vents or from loci within which numerous vents were thickly clustered in close proximity, there were numberless scattered orifices from which a few eruptions or even a single eruption took place. And these dispersed vents were probably scattered about in the intervals between the central localities of eruption. Such craters would in the lapse of ages be wholly obliterated, and their outpoured masses reduced to mere remnants The general effect of secular decay has been to level Ihe volcanic piles and build up the lowlands with the debris. On the other hand, the great faults have brought up to daylight masses of bedded lavas which otherwise would have been concealed, and erosion has in many places attacked the faulted edges of the upraised blocks and sawed deep ravines and chasms in which the igneous masses are tolerably well displayed. Thus we are enabled to gain information concerning the location of the centers of eruption which would otherwise have been unattainable. But the knowledge so gained is far less perfect than is desirable. Although it may seem that an investigation of such importance ought to be easy, it is by no means so. The vastness of the masses displayed at any center of eruption is such that no conception of their totality or of their general arrangement can be gained without a somewhat protracted investigation of a large area. But so rugged and formidable are the physical features that such an investigation is about as difficult an undertaking as ever falls to the lot of a geologist. The petrographic work has not been embodied in this volume. It has not yet been completed, though considerable progress has been made. Yet if it had been practicable to obtain the means to prosecute this branch of research to the end, and to publish the results in such form and with such illustration as the scientific student of the present day demands, it would have been done. It was originally intended to make a thorough series of chemical analyses of the volcanic rocks of this -district. Many |
