will ensue, but whether at different geological eras the physical conditions attending the cooling of eruptive masses have been substantially identical. That this has sometimes been the case will scarcely be denied. The problem with which geologists have to deal, however, is not precisely that just stated, for, since the earlier formations have been deeply eroded, while the degradation of comparatively recent rocks is, as a rule, correspondingly small, upper portions of more recent eruptions have to be compared with lower portions of more ancient eruptions. The lithological problem is, therefore, greatly complicated. The main purpose of lithology being, as the author believes, to trace the physical conditions through which a mass of readily ascertainable chemical composition has passed, in the present state of ignorance concerning the effects of temperatures and pressures, the most rational method is to study and record every peculiarity of every occurrence and every perceptible difference between rocks. . . . Rocks can, of course, never be classified with the sharpness of minerals. Rocks are essentially mixtures, and therefore pass into one another insensibly. The wonder is, that rocks not only conform in some degree to a system, but that certain lithological types exhibit such an extraordinary persistence, being met with at the most remote quarters in typical development. While the very nature of things thus excludes a rigid classification of rocks, observation clearly indicates the possibility of reducing them to a natural system.
Unfounded Fears.—Something is occasionally said about the expediency of avoiding heavy work in very warm weather. The "Lancet" evidently thinks there has been too much talk of the kind, and expresses views of its own on the matter. It says: "After it has been proved to demonstration that nothing whatever should be done, in schools, particularly, during hot weather, we shall be quite prepared to make common cause with the school-children in the endeavor to demonstrate that it is undesirable to do anything in cold weather either. In short, let us give up eating, drinking, working, playing, sleeping, and living, in the last issue, lest some harm should befall. It is rank nonsense to write and speak, as too many who ought to be better informed are writing and speaking just now, as though every act of existence were beset with peril. It would be better for all of us and the world at large if less ingenuity were spent on the invention of scares, and in the pitiful task of working on the fears of the public."
Caries of the Teeth.—Caries of the teeth has been but little investigated scientifically, according to Professor Busch, of Berlin, because in studying it the external hurtful processes have been alone considered, whereas the second important factor, the resisting power of the teeth, has been overlooked. It appears to be peculiar to man, having been observed in no animal; but not to be characteristic of civilized man alone, for it has been observed in large collections of skulls, including those of prehistoric time. Some races are more disposed to it than others. Certain families are particularly predisposed to it. General habit of body has a pronounced influence upon its development, as well as certain peculiarities and crises of physical condition and disease. Disposition to caries shows itself even in the developing tooth in the composition of its enamel, which is undulating, whereas teeth with quite smooth enamel have much greater power of resistance. The enamel appears to be the only tissue in the body which is subject to no metabolism, and which remains quite unchanged. Every alteration in it which is caused by external influences, and every defect of the enamel, remains during the whole of life, and can never be repaired. Dentine also shows differences in structure as regards its disposition to caries. The dentine tubes cither run regularly closely side by side, when the teeth have greater power of resistance; or they branch and surround cellular bodies, or even small air-vesicles, when the teeth fall an easy prey to caries. If dentine has been decalcified at any place by the action of acids, it undergoes putrefaction under the influence of bacteria which do not seem to belong to any particular species. Dentine is sensitive, though nerve-filaments have not as yet been traced into it. Actual toothache does not occur in the course of
