22 THE MINERALS CONSTITUTUSFG EOCKS 



but is rarely met with in other rocks, and is wholly wanting in 

 the basic eruptives. Sericite is a silvery white, or greenish, 

 hydrous, secondary mica occurring commonly as an alteration 

 product from feldspar. Lepidolite, a lithia mica of a white or 

 faint pink color, is frequently found in pegmatitic veins in the 

 older rocks. 



Biotite, the black iron mica, is a silicate of alumina, iron, and 

 magnesia, and is much more general in its distribution than is 

 muscovite. It undergoes alteration into chloritic and ferrugi- 

 nous products and is often an important feature in hastening 

 rock disintegration. Other black micas, sometimes distinguish- 

 able from biotite only by chemical means, are lepidomelane and 

 houghtonite. A pearl gray potash mica, phlogopite, is an im- 

 portant constituent of many limestones, as in northern New 

 York and adjacent portions of Canada. 



All micas, owing to their eminently fissile structure, allow the 

 ready percolation of moisture, and hence, though in themselves 

 of difficult solubility, are elements of weakness in any stone of 

 which they may form a part. The characteristic form of de- 

 composition begins as in other silicate minerals, with hydration. 

 This in the dark varieties is accompanied by a higher oxidation 

 of the iron. The folise gradually lose their elasticity and crumble 

 away, the bases being removed in solution. The complete de- 

 composition of the micas is, however, brought about very slowly, 

 and almost any granitic soil, however thoroughly decomposed, 

 will, on washing, show small flakes of the mineral still remaining. 

 However rusty, too, these may appear, a little hydrochloric acid 

 cleans them up, showing remnant shreds still readily recog- 

 nizable. For some unexplained reason those granitic rocks 

 containing a considerable proportion of white mica are almost 

 invariably more friable and easily disintegrated than those con- 

 taining biotite. 



Olivine ( Chrysolite, Peridote) . — Composition : Silicate of 

 iron and magnesia, (MgFe) 08104. 



This is an essential constituent of basalt, dunite, imburgite, 

 Iherzolite, and pikrite, and a prominent ingredient of many 

 lavas, diabases, gabbros, and other igneous rocks. It also occurs 

 occasionally in metamorphie rocks and is a constituent of most 

 meteorites. Olivine is subject to extensive alteration, becom- 

 ing changed by hydration into serpentine or talcose and chloritic 

 products, with the separation of free iron oxides. It occurs in 



