Mr. Hills’s paper suggests a number of questions, some of which can be fairly definitely answered, while others seem to me to be worth recording on the principle that the best way to get a question answered is to ask it. The separation of the granitic material into limited regions of the earth’s surface to form the continents could clearly not occur when the earth was fluid, and after the earth solidified any mechanical cause would meet formidable difficulties. Explanations seem to be restricted to (1) a failure or partial failure of magmatic differentiation in the oceanic regions, so that the granite has separated out since solidification in the continental regions, but remained dissolved in the oceanic ones, (2) some mechanical process arising during solidification. There are difficulties about both possibilities. Mr. Hills’s theory belongs to the second class, and I think needs serious consideration.