In many instances limestone has been metamorphosed, as will be described below, and converted into crystalline marble. Commercially, however, all limestone, whether crystalline or not, which is susceptible of a polish, is termed marble. Under certain conditions calcium carbonate is concentrated at or near the earth's surface by chemical agencies, as about springs where calcareous tufa, travertine, etc., are precipitated, and in caverns where stalactites and stalagmites are formed. Stalagmite sheets are sometimes composed of variegated, laminated layers, and when polished produce a beautiful decorative stone which passes under the name of onyx marble. Deposits of this character of commercial importance occur in Arizona and Mexico. Calcium carbonate concentrated in lakes through the combined action of chemical and vital agencies produces the so-called marl, now extensively utilized in the manufacture of Portland cement. In this mode of accumulation the calcium carbonate is dissolved by percolating waters from the rocks and soils and carried to lakes in solution; it is there precipitated largely through the vital action of certain algae and deposited as a fine white ooze. Thousands of deposits of this nature, varying in extent up to several hundred acres, and having a depth of from a few feet to 40 and even 60 or more feet, occur in the portion of the continent covered with glacial drift, and especially in the States from New England to Minnesota. The reasons for the greater abundance of marl in this region than elsewhere are that the glacial drift is there highly calcareous, numerous lakes are present, and the climatic conditions are such as to favour the growth of certain aquatic plants, and especially the _Characeae_ or stoneworts, which have the property of eliminating calcium carbonate from ordinary lake waters. The importance of the vital agencies in concentrating substances of economic value is illustrated by the manner in which coal, petroleum, and natural or rock-gas, etc., have been formed.