These limestones present an immense variety, according to the nature of the originating organisms, and the amount of foreign material brought down into the water where they accumulated. The calcareous remains of Chara may form a white deposit on the floors of freshwater lakes. The part played by calcareous algæ in the formation of marine limestones has long been recognised; but the detailed exploration in 1904 of the atoll of Funafuti in the Pacific showed that Halimeda may be responsible for a considerable portion of an ordinary "coral-reef." Lithothamnium occurs in immense quantities, associated with molluscan remains, near many shores, and forms a large part of the material of the raised beaches in Spitsbergen. Animal, not vegetable, activity, however, is responsible for the majority of our limestones, and the humbler organisms, by reason of their abundance, play a prominent part in rock-formation. Analogies between the Globigerina-ooze of deep waters and the groundwork of the soft white limestone known as _Chalk_ have been freely pointed out. Early in the nineteenth century, Ehrenberg, in a series of researches with the microscope, proved the organic origin of the compact ground of marine limestones. The occurrence of foraminifera from the shore outwards to truly oceanic waters provides a fine-grained calcareous material which forms deposits at very various depths. The milioline types, often with a surface like that of glazed porcelain, are common in the sandy beds formed near a coast. Few rocks are more fascinating under the microscope than those in which such types are seen in section, associated with detrital grains of quartz, washed down from the land, and perhaps with bright green grains of the marine mineral, glauconite. In Ireland white chalks occur, speckled throughout with glauconite, which looks dark in the rock-mass, but which reveals its green tint when streaked out by the hammer. When formed still farther from land, pure chalk arises from the consolidation of foraminiferal ooze, and the probable depth in which it accumulated must be judged from the nature of the associated organisms. A white limestone may, however, arise in a comparatively shallow sea, where the rivers bring down little solid matter from the land. A coast formed of pure limestone, with clear streams flowing from a land of similar rock behind, may allow of the development of pure limestone on its shores. It is generally agreed that the Upper Chalk of the British Isles and of northern France was laid down in water one thousand fathoms or more in depth; yet the corresponding white limestone of northern Ireland in places follows rapidly on conglomeratic and glauconitic deposits, and seems to owe its purity to the comparative absence of rain and rivers on the highland of crystalline rocks which stretched westward from its shore.