In the Rocky Mountain region, oil ranging in gravity from 13° to 75° Bé. occurs in commercial quantities in formations ranging in age from Carboniferous to Tertiary. Although the Carboniferous oils are predominantly low in gravity and the post-Carboniferous oils high in gravity, there is a great variation in the character of the oil in the same formation in different fields and in different formations in the same field. It is evident that the quality of the Carboniferous oil bears no demonstrable relation to the dynamic metamorphism which the region has locally undergone.
Carbon ratios as ordinarily calculated do not show satisfactorily the rank of the low-rank Cretaceous and Tertiary coals found in the Rocky Mountain region, and are a very doubtful measure of the dynamic metamorphism the coals and enclosing strata have undergone.