others are openly opposed. The latter, however, mostly belong to a school of thought which, chemically speaking, is obsolescent; and by refusing to accept the later notation they bar the doors of progress against themselves. They discover details, but they develop no principles. A reasonable distrust of novelty, however, is always legitimate; and the question may fairly be raised whether the methods of reasoning which are valid in organic chemistry can safely be applied to mineralogic research. The organic chemist deals-with compounds for which the starting-points are simple and well known; in many cases he can determine molecular weights with ease; and his material is so plastic that it can be altered, built up, or transferred in readily traceable ways. Every step in his processes can be followed, and his results may be checked from many sides. Minerals, on the other hand, are hard and stubborn; they form slowly and change with difficulty; they can not be handled as systematically as their organic analogues, and the evidence concerning their chemical structure is therefore less complete and convincing. Still, the case is not quite hopeless, and much positive work may be done.
Just at this point the main lines of mineralogic investigation seem to converge toward the central stem of growth. Leaving out of account mere questions of descriptive detail, the raw material of scientific thought, we may consider three great divisions of study which touch the problem of chemical structure. First of all, we have the branch of associative mineralogy. Minerals do not occur together at random, in all conceivable groupings, but only in accordance with definite laws which are now subjects for investigation. We can not clearly formulate these laws as yet, but we are learning much about them empirically; so that in many cases, upon finding one species, we instinctively look for certain others, which we are quite sure must exist with or near it. Some minerals are found in granite veins, some in volcanic rocks, and some only in ore-bodies, and each one may be evidence for its neighbors. The chief work of the lithologist is in a limited portion of this field; for he considers the minerals which are aggregated into rock-masses, which latter represent definite and frequently recurring associations exhibited upon a large scale. The very classification of the rocks is based upon their mineralogical characteristics. Lithology, however, takes into account only a small minority of known species.
Now each well-established group of mineral associates indicates something relative to their origin. It represents the collective conditions under which they came into existence, and points distinctly toward the chemical reactions which formed them. If we study any one locality closely, we shall discover some details of curious significance. Some minerals occur enveloped by, inclosed in, or implanted upon others; some line cavities, and some represent incompleted processes. We see clearly that one was formed before or after another;