It is hardly necessary to point out how very important it is that those on whom falls the onus of naming a rock should decide which is the most fitting name only after very careful consideration. Some there are who profess to be above the petty worries of nomenclature, and from these only a loose application of rock names can be expected. This is liable to mislead those dealing with problems of petrogenesis, who are justified in assuming that those competent to carry out geological research are also competent to apply correct names to their discoveries. That this is often not the case can be easily demonstrated from quite recent papers. The difficulty arises from two causes: the lack of a system of rock classification which in any sense approaches the ideal, and a desire for brevity. Concerning the first difficulty it is sufficient to say that little advance in placing the nomenclature of rocks upon a firm and common basis is possible until one universally recognized system of rock classification is in use. It is still open for investigators to choose, without any set standard, which of several variable characters of a rock should be the criterion upon which the systematic position of the rock depends. Thus, one writer places mode of occurrence before micro-structure, while his neighbour deems it wise completely to ignore mode of occurrence and makes micro-structure the deciding factor.