Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: about Paris, and it forms the best building stone in that neighbourhood. The cellular spaces represent the casts of fossil shells, etc. Silicified wood is a comparatively rare form of chalcedony, and shows the microscopic details of vegetable structure. Tree trunks over a yard in diameter have been found. Sand is an accumulation of grains of mineral matter derived from the disintegration of rocks. When rocks or minerals are broken up by either natural or artificial agencies, the products may be gravels, sands, and muds or clays, according to the size of the constituent particles. When many of the particles are as large as peas (or larger) the product is a gravel, and when the material forms an impalpable powder owing to the extreme fineness of the particles (about TTHfo in. across), it is a mud or clay ; intermediate grades constitute sands, which may vary greatly both as regards size of grains and the nature of the component minerals. Sands which have been sorted out by gentle currents of water, or by steadily blowing winds across smooth arid lands, may be uniform. Natural sands are mostly formed by the action of the air, rain, frost, heat, plants, and other agencies in breaking up the surfaces of rocks, but volcanic sands (consisting of fine particles of lava) were produced by explosions of steam in the craters of active volcanoes. A few sands are artificial, like the crushed tailings formed in the extraction of metals from their ores. Quartz is the most abundant rock-forming mineral, and its hardness, want of cleavage, and chemical properties enable it to offer a strong resistance to both mechanical and chemical action ; it survives after the destruction and dispersal of the accompanying minerals, and accordingly sands are generally silicious. Much of the earth's surface ... --This text refers to an alternate Paperback edition.