Note on Yugasima Group Exposed near Syuzenzi, Izu

The purpose of this study was to determine the stratigraphic relation between Lepidocyclina limestones exposed at Simosiraiwa and Ono, 5 km., E and NE respectively of Syuzenzi, Izu, and the Yagasima Group which from the foundation of Izu penisula., Tertiary rocks exposed in the area studied consist of a complex of pyroxene-andesites and dacite tuffs., In the latter complex are included flows of pyroxene-andesites and dacites., The limestones occur in the apparently upper horizon of the dacite tuff complex., Generally speaking, the dacite tuff complex overlies that of pyroxene-andesites., At the junction of the two complexes, andesitic and dacitic pyroclastics grade to one another., Again, layers of dacite tuff are sometimes intercalated between lavas and pyroclastics of the andesite complex., These facts, taken in conjunction with the similarity of the nature of hydrothermal alteration of the rocks of the two complexes, strongly indicate that the andesite and dacite complexes were deposited successively., In the limestones we found fossils listed in Table 2, from which the age of the dacite tuffs is determined as older Miocene, or F1 according to the "letter nomination" recently proposed by N., IKEBE, K., KANEHARA, and K., HUZIWARA., The andesite complex may also belong to the same general age., The andesite complex resembles closely the Yugasima Group exposed at the type locality Yugasima, 9 km., S of Syuzenzi, especially in that the rocks suffer propylitization., There is no doubt that the andesite complex of the area studied belong to the same group., The age of the Yugasima Group is therefore olderMinocene or F1/