Quartz has an economic value directly in glass sand and of course as a soil constituent. In the latter capacity, it is taken up by many plants, and is the silica that studs the saw edges of the blades of sedges and grasses. The precious stones, agate, amethyst, and jasper are varieties of quartz. [Illustration: QUARTZ AND SILICATES. 3/4 Life-size. FROM COL. CHI. ACAD. SCIENCES. A. W. MUMFORD, PUBLISHER, CHICAGO. COPYRIGHT 1900, BY NATURE STUDY PUB. CO., CHICAGO. 1--Asbestus. 2--Feldspar. 3--Quartz Crystal. 4--Small Garnets in rock. 5--Garnet. 6--Opal. 7--Smoky Quartz.] The silicon that is so important a constituent of quartz, composes with aluminum a large part of various minerals comprised under the name feldspar. This substance is slightly less hard than quartz and has many variations in color; but, unlike quartz, shows regular cleavage faces. Feldspar is always crystalline, but good crystals are not common. It is very difficultly soluble, yet readily yields to the influence of weathering. A feldspathic rock hence readily crumbles. During the process of disintegration, the feldspar may change from a clear, hard, glassy mineral to a dull, opaque substance. This product of disintegration is our common white clay. With quartz, then, feldspar is of great importance in the forming of soil. Allied to the feldspar group of minerals as regards cleavage, and yet of far different special characteristics is the class of substances known as mica. How many of us ever think of the so-called isinglass of our stove doors as a mineral substance? Yet transparent mica, muscovite, is the source of that household convenience. A study of the specimens of mica in our stove door will provide abundant ideas of the nature of mica. We have often noticed how, under the influence of excessive heat, the isinglass splits into thin sheets, thus showing the cleavage of the mineral. These plates of mica are of especial value in giving cleavage to rocks which would otherwise fracture irregularly. The cleavage of slates and of the common shale rocks is due to the presence of mica particles which have, at some period in the history of the earth's crust, through the action of heat and pressure, been arranged along definite planes. Isinglass represents the transparent variety of mica. Other varieties are brown and even black, owing to the presence of traces of potassium, magnesium, iron, etc., in varying degree. Some micas do not easily decay, and so we frequently see glittering particles among the fine grains of soil and the sands of beaches.