We encounter, in thin sections of both, volcanic and metamorphic rocks, microscopic crystals arrested in every stage of their growth, and it is not true that these earlier forms are mere epitomes of the perfected individual. We have the fundamental globulite and the complicated and fantastic "growth-forms," which are as different from the finished crystal as is the larva from the butterfly. Thus, to one familiar with such facts as these, there can be no confusion in speaking of the "embryology of a crystal." We think with wonder of the marvelous vitality of seeds which sprouted after three thousand years spent in Egyptian pyramids, and yet the "vitality" of a crystal is such that it will continue its growth under favorable conditions after any number of thousands of years of interruption.
There is, however, nothing among the recent disclosures of the microscope in regard to rocks so surprising as their delicate adjustment to their environment. We are accustomed to look upon the masses of our mountains as the very type of what is stationary and eternal; but in reality they are vast chemical laboratories full of activity and constant change. With every alteration of external conditions or environment, what was a state of stable equilibrium for atoms or molecules ceases to be so. Old unions are ever being broken down and new ones formed. Life in our planet, like life in ourselves, rests fundamentally on chemical action. The vital fluid circulates unceasingly through the arteries of the oceans and the currents of the air; it penetrates the rocks through the finest fissures and invisible cracks, as the human blood penetrates the tissues between artery and vein, producing, with the help of heat and pressure, like changes in the histology of the globe. The recurrence, after a long interval, of the same set of conditions in the same rock-mass, may bring about the unending cycle—analogous to succeeding generations—which Hutton, the earliest of the Scotch geologists, recognized a hundred years ago.
Such processes as these, which properly represent the physiology of our earth's crust, have long been suspected, but their exact nature and details are only now being gradually disclosed by microscopical studies of the rocks.
Suppose, for instance, that a lava-stream bursts from the side of some volcano. As it flows onward, quickly solidifying and crystallizing under circumstances of intense heat, chemical compounds are produced which accord with such conditions, but perhaps not with those ordinarily obtaining at the earth's surface. If this is the case, the hardened lava will be in a chemically unstable state, and will tend in turn to adapt itself to its new surroundings by chemical change.
Countless examples of this adaptability of rocks to their envi-
