IT is well that Dr. S. C. Lind directs attention to certain points in his theories which it was not possible within a limited space to deal with in my recent notice of his work, if only to promote a wider interest in the potentiality of radioactive stress in determining the trend of development and composition of petroleum. The hydrogen question is certainly a difficult one to dispose of, especially in view of the comparative rarity of this gas in a free state in natural gas, a point I have been at pains to verify from diverse published records, but Dr. Lind's suggestions are helpful, if not actually convincing. One would like to know, however, what sort of catalysts he has in mind which might be presumed to exist in the oil environment and to be capable of promoting reaction whereby the hydrogen is eliminated. Are they organic or inorganic? If the former, can we invoke bacterial aid, in which case the agent is doubtfully catalytic? If the latter, would not careful analysis of the source-rock reveal some competent mineral substance?