Contents Page Table 1 1. Average chemical analyses of certain types of plutonic igneous rocks (R. A. Daly) 2 2. Approximate mineral compositions of principal types of plutonic igneous rocks (E. S. Larsen) 3 3. Composition of stony meteorites (L. LaForge) 4 4. Composition of iron meteorites and meteoritic iron (L. LaForge) 5 R emarks : Each average composition was computed from single analyses of specimens to which the respective reporters gave the name printed at the top of the column. The average may thus be regarded as at the center of gravity of opinion as to the composition of rocks which may properly be described by this name. For many purposes, these average compositions may be taken as adequately descriptive of the materials falling into the same classifications in other tables; for other purposes, the analysis of the individual specimens on which measurements have been made is essential. Since most of the analysts neglected the effect of the adsorption of water by their specimens when pulverized, the proportion given for this oxide is in general somewhat too high. The analyses of significantly weathered specimens were excluded, insofar as possible, from the data finally used. On the whole, about three-fourths of the total water indicated was driven off at temperatures no higher than 105°C. It should be noted that the percentage of TiO 2 in many of the older analyses is too low. This error of itself would make the reported percentages of Al 2 O 3 slightly too high. The average composition of the . . .