These great earth-movements are not always perfectly smooth and steady, but they are accompanied by slipping or crushing now and then; and, as a result of the shock thus produced, a swift vibratory movement or jar, which we know as an _earthquake_, runs through the earth’s crust. Extensive fissures are also formed, opening down to the regions where the rocks are liquid or plastic, and through these the melted rocks flow up to or toward the surface. That portion which flows out on the surface builds up a volcanic cone, while that which cools and solidifies below the surface, in the fissures, forms dikes. Thus among the igneous or eruptive rocks we have two great classes,—the _dike_ rocks and the _volcanic_ rocks. It is important to observe that all these subterranean operations—the formation of continents, of mountain-ranges with all their attendant phenomena of folds, faults and cleavage, and every form and phase of earthquake and volcanic activity—depend upon or originate in the interior heat of the earth. And over against the _superficial_ or _aqueous_ agencies, originating in the _solar_ heat and producing the _stratified_ or _sedimentary_ rocks, we set the _subterranean_ or _igneous_ agencies originating in the _central_ heat, and producing the _unstratified_ or _eruptive_ rocks. ------------------------------------------------------------------------ STRUCTURAL GEOLOGY. [Illustration] In geology, just as in biology, there are two ways of studying structure,—the small way and the large way. In the case of an organism, we may select a single part or organ, and, disregarding its external form and relations to other parts, observe its composition and minute structure, the various forms and arrangements of the cells, etc. This is histology, and it is the complement of that larger method of studying structure which is ordinarily understood by anatomy. The divisions of structural geology corresponding to histology and anatomy are _lithology_ and _petrology_. Lithology is an in-door science; we use the microscope largely, and work with hand specimens or thin sections of the rocks, observing the composition and those small structural features which go under the general name of texture. In petrology, on the other hand, we consider the larger kinds of rock-structure, such as stratification, jointing, folds, faults, cleavage, etc.; and it is essentially an out-door science, since to study it to the best advantage we must have, not hand specimens, but ledges, cliffs, railway-cuttings, gorges, and mountains. LITHOLOGY.