COSMICAL GEOLOGY.
A full treatment of this phase of geology is out of place in a brief general article. Moreover, much of it belongs to astronomy. Studies of the shape of the earth, and the resemblances between the earth and other bodies in space, both in form and composition, are undertaken by physicists and astronomers. These studies, however, throw light upon the earliest phases of earth history. pointing to the conclusion that the earth, like other bodies in space, was once a molten sphere which has cooled on the outside, forming a solid, cold crust. Of the original crust geo logical investigation has as yet found no sign. 1 t is to the continued cooling of this once molten sphere that we owe some of our most important geological events. The forces, having their scat in the heated interior, may be considered as terrestrial or hypogene forces. The passage of light and heat to the earth, the great movements of rotation and revolution, and the pull exerted by the sun and moon, constitute the restrial or epigene forces, which, aided by gravity, and acting through the medium of air and ocean, set in motion another series of geological agencies.
Dynamic geology is concerned in a study of the operations of these two sets of forces whose origin is cosmical.
Other phenomena of the earth having an in fluence on geological history are the precession of the equinoxes and the variations in the eccen tricity of the earth's orbit. These two astro nomical changes have influenced the amount and distribution of beat on the earth's surface in past times. but to what extent is an unsolved problem. There are still other obscure questions in cosmical geology; for example, the possible changes of the earth's axis and centre of gravity. Being on the borderland of two or three sciences, and dealing with subjects on which it is difficult to gather facts, these are among the great scien tific problems awaiting solution. •