Page:Popular Science Monthly Volume 39.djvu/598

580 a clew to what happens when water freezes and.melts or when it is turned into steam! It is only within fifty years that physics and chemistry have begun to assume the form of coherent bodies of scientific truth. Evidently geology could not be expected to take scientific shape until late in the eighteenth century, or to make any notable conquests before the nineteenth. But when geology did win its first great triumph, about sixty years ago, it was in some ways the most remarkable moment in the history of thought since the promulgation of the Newtonian astronomy. Newton proved that the forces which keep the planets in their orbits are not strange or supernatural forces, but just such forces as we are familiar with on this earth every moment of our lives. Geologists before Lyell had been led to the conclusion that the general aspect of the earth's surface with which we are familiar is by no means its primitive or its permanent aspect, but that there has been a succession of ages in which the relations of land and water, of mountain and plain have varied to a very considerable extent, in which soils and climates have undergone most complicated vicissitudes, and in which the earth's vegetable products and its animal populations have again and again assumed new forms while the old forms have passed away. In order to account for such wholesale changes, geologists were at first disposed to imagine violent catastrophes brought about by strange agencies—agencies which were perhaps not exactly supernatural, but in some unspecified way different from the agencies that are now at work in the visible and familiar order of Nature. But Lyell proved that the very same kind of physical processes which are now going on about us would suffice during a long period of time to produce the changes in the inorganic world which distinguish one geological period from another. Here, in Lyell's geological investigations, there was for the first time due attention paid to the immense importance of the prolonged and cumulative action of slight and unobtrusive causes. The continual dropping that wears away stones might have served as a text for the whole series of beautiful researches of which he first summed up the results in 1830. As astronomy was steadily advancing toward the proof that in the remotest abysses of space the physical forces at work are the same as terrestrial forces; so now geology, in carrying us back to enormously remote periods of time, began to teach that the forces at work have all along been the same forces that are at work now. In that early stage when the earth's crust was in process of formation, when the temperature was excessively high, there were, of course, phenomena such as can not now be witnessed here, but to find a parallel to which we must look to certain other planets—such as violent atmospheric disturbances, and such as the dissociation of chemical elements which we are accustomed to find in