Dr. Harker has familiarized us with the use of variation diagrams, in which the Pereentage amounts of a constituent are plotted against the amount of silica present. He applied these diagrams chiefly to study selected rock groups, but also gave certain generalized eurves fo illustrate the chemical properties of his 'Atlantic' and 'Pacific' facies. Such variation diagrams based on an adequate body of analytical data form a useful means of constructing diagrams for the comparison of rock analyses. Some means of generalizing analyses becomes a necessity as their number accumulates and the labour of comparing individual analyses increases.