Any science which cannot express its conclusions by means of number must still be regarded as in its infancy. Such an expression is the only test of precision and accuracy of thought on the part of its workers, and such important characters only develop, as in the human mind, when the romance and rashness of youth have given place to the more cautious and matter-of-fact features of maturity. It is therefore a sign of healthy development when those engaged in the study of petrology begin to turn their attention towards careful numerical calculation as a basis of classification.