Abstract
The rise in the rank of coal toward the north where the Ishikari group increases in total thickness within the western wing of the Sorachi anticline have been recognized, and this tendency could be applied to the Noborikawa coal-bearing formation, the lowest of the Ishikari group, and to the Bibai coal-bearing formation that lies about the middle of the group, both of which are the major coal-bearing formations in Sunagawa region, the central part of the above mentioned.
The increase in C percentage per 100 meter coverage of coal seams have been clarified to be 1. 5% in average when calculated individually in the two coal-bearing formations. Consequently, the depth of the coverage is considered to be major factor holding a prominent position in the T. P. conditions in accelerating the degree of coalification. However, should it be so, Noborikawa, in contrast with Bibai, should be much higher in rank.
The cause of the approximation of the rank of coal in the two coal-bearing formations is discussed geologically in the text, and it could be considered that the thickness of each formation thins toward the axis of the Sorachi anticline, resulting in the coverage of the Noborikawa coal-bearing formation becoming near equal to that of the Bibai coal-bearing formation.
