Abstract
The numerous dolerites of various types are seen on the Dewa Hills, Northern Honshû. They are the Hirataki, Hatamura, and Nakada types, being lithologically olivine dolerite, hypersthene dolerite with olivine and hypersthene dolerite without olivine respectively.
The hypersthene in the Hirataki dolerite is found in the pyroxene-rich layer in the lower part of the si1l due to the gravitational differentiation, while the hypersthene in the Hatamura and Nakada types is included throughout the whole mass.
The writer has examined the genesis of hypersthene dolerite on the basis of the micrometrical and chemical analysis, concerning the pyroxene rich layer of the differentiated products. It is concluded that the hypersthene dolerite is not derived from the same process as is seen in the Hirataki dolerite due to the gravitational differentiation, but characterized by an increase of SiO2 and A12O3 as well as a decrease of MgO.
