_Conglomerate_ consists of water-worn pebbles of various kinds and sizes cemented together, and sometimes making a strong, compact rock. _The limestone formations_ are extensive in nearly all countries. In their structure some are very compact and break with a smooth surface. Those capable of a fine polish are called marble, the more common uses of which are well known. The purest crystalline limestone is used in sculpture; the best quality being obtained from Carrara, Italy, and that called Parian from the island Paros. _Chalk_, a useful formation, is a carbonate of lime. In some caves the dropping of calcareous water forms stalactites, which hang from the roof like immense icicles, and are often extended till they meet the accumulations below, called stalagmites, and form beautiful columns. Of the more than seven hundred crystals from this source alone, and of the many other varieties of minerals having much in common, and yet enough that is peculiar to distinguish them, no mention can be made. A careful reader and close observer will gather from familiar objects a fund of information of great value. The parallel strata mentioned are not always horizontal, but sometimes nearly, if not quite perpendicular. Occasionally a ledge broken quite through separates, and the rock on one side of the fissure is either elevated or depressed, making what is called a fault. The fissures crossing a bed of rock are often filled with a mineral entirely different from the rock itself. In some cases where the vein is small the foreign substance may have come in from above or laterally, deposited from water as in the case of stalactites. The larger fissures were evidently filled with the melted material thrust up from beneath. The unstratified rocks are in masses, without fossils of animals or plants, and of igneous origin. Some of this class were probably formed later, and by the melting of secondary rocks, but most of them by the gradual cooling of the central mass containing the melted minerals embodied in them. DYNAMIC GEOLOGY