Page:The Journal of geology (IA journalofgeology21894univers).pdf/182
by the vessels sent out on scientific expeditions by the various governments of Europe and our own. The most important of these expeditions has been that of the Challenger, sent out by the British government in the years 1872-76. In the report upon the Deep Sea Deposits recently published as a result of that expedition, Professors Murray and Renard, the authors, present the latest results upon the character and distribution of greensand, and, at the same time, propose a theory to account for the chemical changes which have taken place to produce the mineral glauconite which characterizes all greensand deposits.
A typical greensand is composed of glauconite associated with greater or less amounts of land-derived material. Among the more common minerals thus found are quartz, feldspar, hornblende, magnetite, augite, zircon, epidote, tourmaline and garnet, together with fragments of the continental rocks, such as gneiss, mica-shist, granite, diabase, etc. A variable amount of calcareous matter derived from the shells of organisms is also present.
The glauconite occurs in minute grains, seldom exceeding 1 mm. in diamter, although they may become agglomerated into nodules several centimeters in diameters by means of a phosphatic cement. The grains are always more or less rounded, and at times mammillated, with irregular surface outline. They are generally black or dark green in color, but become brighter green upon being crushed. The surface of the grains is sometimes covered with fine punctures, while at other times it is smooth and shining. Some of these glauconitic grains are distinct internal casts of foraminifera, and of other calcareous shells, but more often they only reproduce indistinctly the form of the chambers, or show no definite connection with the organisms in which they originated.
It is estimated that greensand deposits cover approximately 1,000,000 square miles of the sea floor. They are found limited to those portions adjacent to the coasts, and, for the most part, along the higher parts of the continental slopes where land-derived materials are deposited in perceptible, yet small amounts. The production of glauconite seldom reaches to greater depths