The accompanying two maps (in pocket) show the distribution of earthquake epicenters in the Philippine Archipelago as plotted by the Rev. William C. Repetti, S.J., Chief of the Section of Seismology and Terrestrial Magnetism of the Manila Observatory, for the years 1920 to 1936, inclusive. The text presents their relations to active structures of several types observed by the writer in a geological reconnaissance in the spring of 1937. The dynamics of the Archipelago and the mechanisms of the stresses are sketched in accordance with the hypothesis that the Philippine block is an erupted mass rising along the contact between the crustal elements that respectively underlie the China Sea and the adjacent Pacific Basin.

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