I THANK Mr. Bonacina for his sympathetic comment in NATURE of September 22, p. 436, on my letter on “Polar Temperatures and Coal Measures” and for the added clarity he has brought to this subject. He mentions disagreement with me on one minor issue only, and that relating to the south polar regions. In that connexion I am glad of the opportunity to confess that my thinking on the subject of polar coal measures has really been based almost exclusively upon my knowledge of the Arctic. My suggestion that similar conditions might explain Antarctic coal was a sort of parenthetical remark made without any special consideration of the Antarctic problem.