In general the qualities of the earth materials which determine their availability for use are only to a minor extent the qualities which the geologist ordinarily considers for mapping and descriptive purposes. The usual geological map and report on a district indicate the distribution and general nature of the common rocks, and also the extent to which they are being used as mineral resources. Seldom, however, is there added a sufficiently precise description, for instance of a clay, to enable the reader to determine which, if any, of the many different uses the material might be put to. The variety of uses is so great, and the technical requirements for different purposes are so varied and so variable, that it is almost impossible to make a description which is sufficiently comprehensive, and at the same time sufficiently exact, to give all the information desired for economic purposes. If the geologist is interested in disclosing the commercial possibilities in the raw materials of an area, he may select some of the more promising features and subject them to the technical analysis necessary to determine their availability for special uses. In this phase of his work he may find it necessary to enlist the cooperation of skilled technicians and laboratories in the various special fields. The problem is simplified if the geologist is hunting for a particular material for a specific purpose, for then he fortifies himself with a knowledge of the particular qualities needed and directs his field and laboratory study accordingly. Too often the geologist fails to recognize the complexity and definiteness of the qualities required, and makes statements and recommendations on the use of raw materials based on somewhat general geologic observations. On the other hand, the engineer, or the manufacturer, or the builder often goes wrong and spends money needlessly, by failing to take into consideration general geologic features which may be very helpful in determining the distribution, amount, and general characters of the raw materials needed. It is difficult to draw the line between the proper fields of the geologist and those of the engineer, the metallurgist, and other technicians. It is highly desirable that the specialist in any one of these fields know at least of the existence of the other fields and something of their general nature. Too often his actions indicate he is not acutely conscious even of the existence of these related branches of knowledge. The extent and detail to which the geologist will familiarize himself with these other fields will of course vary with his training and the circumstances of his work. Whatever his limit is, it should be definitely recognized; his work should be thorough up to this limit and his efforts should not be wasted in fields which he is not best qualified to investigate.