Many conjectures have been formed, and many theories propounded, as to the nature of the internal parts of our globe. Some have supposed that there is an outer solid film or crust, some ten or twenty miles thick, enveloping a vast ball of intensely heated matter; others have attempted to show that the interior must be nearly solid throughout, with, however, great lakes, or vesicles of gas and melted rock, somewhat after the fashion, we may suppose, of the oil-holes in a Gruyère cheese. But whether the heated material occupy the whole or only parts of the internal area, is not of much consequence to the practical geologist; he is content to believe that it exists, and in sufficient quantity, too, to produce the most momentous changes on the surface of the earth. We see the effects of this subterraneous agent in earthquakes and volcanoes, and the geologist can tell us of similar, as well as of other changes, effected by it during past ages. Granite hills, and mountainous districts of mica-slate and gneiss, bear evidence of what is termed _metamorphism_--a change in the mineral structure of rocks, believed to have taken place through the agency of heat deep in the interior of the earth; for no analogous appearances have been detected in progress at the surface. Such rocks, known as _metamorphic_, or _hypogene_, still form a difficult problem, not likely to be satisfactorily solved until the chemist shall have thoroughly investigated the subject; for it seems likely to be found, after all, that long-continued chemical action, without a very alarming degree of heat, may have produced even the most intense metamorphism. But dropping this part of the subject, in which so much yet remains to be discovered, let us look for a little at another branch of the geologist's evidence, where we meet with no such hampering hypotheses and doubtful conjectures, namely, the _trap_-rocks.