n others.

In this text the writer has certainly not escaped the difficulties in
regard to names above referred to, nor in fact has he made any
exceptional effort to do so. His chief purpose is to convey, in somewhat
elementary terms, an understandable idea of the central features of
economic geology. In the main, the most widely accepted terms are used.
Almost at every turn it would be possible, in the interests of
precision, to introduce qualifying discussions of names,--but at the
expense of continuity and perspective in the presentation of the
principal subject-matter. The writer does not wish to minimize the
necessity for careful and precise nomenclature; but he regards it
important that the student focus his attention on the central objective
facts of the subject, and that he do not become misled by the sometimes
over-strenuous advocacy of certain names or classifications in
preference to others. If the facts are understood, he will ordinarily
have no difficulty in judging the significance of the variety of names
proposed to express these facts. If, on the other hand, the student
approaches the subject with a ready-made set of names and definitions
learned by rote, he is in danger of perceiving his facts from one angle
only and through a distorted perspective.

MINERAL DEPOSITS AS MAGMATIC SEGREGATIONS IN IGNEOUS ROCKS

In this class are included deposits which crystallize within the body of
igneous rock, almost, if not quite, simultaneously with the adjacent
rock. These deposits form one of the main types of _syngenetic_
deposits.

The titaniferous magnetites constitute a widely distributed but at
present commercially unavailable class of iron ores. The magnetite
crystals of these deposits interpenetrate with the other constituents of
an igneous rock, commonly of a gabbro type, and the deposits themselves
are essentially igneous rocks. Their shapes are for the most part
irregular, their boundaries ill-defined, and their concentration
varying. While their magmatic origin is clear, there is little agreement
as to the precise conditions which determined their segregation in the
molten rock. There is often a tendency for the ores to follow certain
primary sheeted structures in the igneous mass, a fact for which the
reason is not obvious.

The Sudbury nickel ores, of Ontario, Canada, the principal source of the
world's nickel, lie mainly within and along the lower margin of a great
intrusive igneous mass of a