Natural silicates vary in their O 18 /O 16 ratios by as much as 2-4%. This difference is largely due to an exchange reaction between silica and water during the processes of erosion and sedimentation, as a result of which sedimentary rocks are richer in O 18 than are the igneous rocks. The temperature coefficient for the silica-water exchange has been estimated to be - 0·128%., per degree Centigrade. The O 18 content of basic igneous rocks is low and confined to a narrow range (6·4 to 7·0%.). The alkalic and granitic igneous rocks are richer in O 18 , and their isotopic ratios are more variable (7·7 to 12·2%.). In granitic rocks, quartz shows a greater preference for O 18 than does orthoclase; the distribution coefficient is 1·20 in favor of the quartz. The isotopic constitution of stony meteorites is identical to that of the basic igneous rocks, but tektites resemble the sedimentary rocks in their isotopic composition. The analysis of a granophyregabbro sequence revealed an isotopic gradient which suggests solid diffusion as the mode of origin for the intermediate rock. The mechanism of metamorphism has been studied from the standpoint of isotopic composition. The results are not conclusive, but suggest that metamorphism is associated with a depletion of O 18 .