Of late years a great deal has been written on the genesis of ore-deposits, and in particular much space has been devoted to more or less controversial discussions of the possibility of a magmatic origin for many important types, both in individual instances and by way of generalizations. The widespread acceptance in many quarters of a belief in the derivation of such deposits on a wholesale scale from igneous magmas may perhaps to some extent be regarded as a natural reaction from some of the theories of markedly contrasted character that prevailed almost universally in past times. The “swing of the pendulum” is such a well-known psychological phenomenon that it is unnecessary to enlarge on it here. It is applicable to nearly all branches of knowledge, and of ignorance, and was long ago summarized in the law that action and reaction are equal and opposite. If a theory is pushed too far, it is inevitable that in a while it will be regarded as wholly, or for the most part, erroneous, although it may nevertheless contain a good deal of truth. Most of these difficulties and controversies arise from the ineradicable habit of generalizing too broadly, and the rest of them can generally be traced to misunderstandings as to the precise meaning of the terms employed. In the history of the scientific study of ore deposits both of these sources of error have undoubtedly played an important part.