During the last fifteen years it has been one of the writer's duties to teach the elements of Petrology to students in various branches of Engineering, Mining, Chemistry, Forestry, etc. The amount of time which these students, in their undergraduate course, can devote to the subject is limited and precludes any attempt to give them such instruction in optical-microscopical methods of research as would be worth while. The subject has to be treated from the purely megascopic standpoint, as indeed the vast majority of those who have to deal with rocks in a practical or technical way are also obliged to consider them. In giving this instruction the author has long felt the need of a small, concise and practical treatise in which the rocks and rock-minerals are handled entirely from this megascopic standpoint. In such works as exist either the subject matter has not been brought down to date to express our present knowledge of rocks, or it is treated largely from the microscopical or chemical stand point, or the classifications used are based on microscopical research and are thus not available for ordinary use, or the rocks are treated incidentally with respect to some other main purpose as in works on soils, ore deposits, etc. The present work is an attempt to fill this need which the writer believes is also felt in many other institutions in which similar courses in Petrology are given. In addition to this purpose its scope has also been somewhat enlarged to meet the wants of many who have to consider rocks from the scientific or practical point Of View and who are not in a position to use the microscopical method. It is hoped that it may thus prove of service to field geologists.