In the hypabyssal varieties, as well as in surface flows, an examination of hand-specimens often reveals the presence of larger crystals (usually termed porphyritic crystals) scattered in a ground-mass in which no definite structure is visible except under the microscope. These may in part have been formed before the molten magma of the rock had begun its upward ascent, but in general they and the finer-grained base, have crystallized under similar conditions. Examinations of the ground-mass microscopically also shows it to be entirely crystalline in its nature, but the individual components are of very small size. In the extruded volcanic rocks, on the other hand, the porphyritic crystals are probably of much earlier date than the ground-mass in which they are present, the latter being solidified above-ground, where cooling was rapid, pressure suddenly reduced, and many of the gaseous constituents were free to escape readily. As crystals require a certain temperature and a slow liberation of the heat for their formation, in many volcanic rocks the rapid cooling results in the production of a glass instead of a crystalline aggregate, so that the presence of a glassy matrix is in itself evidence of a quick loss of heat. VII.—THE FORMATION OF METAMORPHIC ROCKS. All materials, both sedimentary and igneous, composing the earth’s crust, are liable to be involved in movements of deformation, becoming subject to pressures and chemical agencies, which are often intense in character and productive of far-reaching internal changes. The result is the formation of a series of rocks, which may be of divers origins, but which having lost their original characteristics, may be grouped as metamorphic. By the action of compression and dislocation not only are the major masses folded or broken, but their individual constituents are brought into new relations both as regards their external conditions and internal molecular arrangement. Where lateral pressure has been active as regards the finer-grained rocks, such as clays, the individual particles arrange themselves with their longer axes at right angles to the pressure, the result being the production of a fissile structure or cleavage, totally distinct from the original bedding-planes of the strata. Clays under such circumstances pass into slates, in which no very definite mineral banding is observed, or into schists, in which there is a tendency for the separate minerals, such as mica and quartz, to have a pseudo-stratified appearance, though the individual layers, if closely examined, are found to be lenticular, not parallel in shape. These schists and slates are developed on a large scale in the Red Sea hills and Sinai. The extent to which compression and tension has been carried in these ancient sedimentary strata is well indicated in Sinai, where rounded pebbles of quartz have been fractured, the two broken parts somewhat separated, and the whole re-cemented in a schistose matrix.