Secondary Minerals.— Minerals of this class have originated as the result of the decom position or alteration of some previously exist ing primary or original mineral. The most noteworthy are: The really important rock-forming am phiboles and pyroxenes are to be found among the orthorhombic and monoclinic varieties. And it is necessary to distinguish here between those which are aluminous and those which are non-aluminons, for the special reason that the former on alteration produce chlorite, while the latter produce serpentine.
Kaolin results from the decomposition of orthoclase and other feldspars by the loss of some of the silica and alkalies, and by the ad dition of water (hydration). Serpentine results from the alteration of olivine and the non aluminous hornblendes and pyroxenes by the loss of some of the magnesia and by an addi tion of water. Talc is formed by the hydration and partial decomposition of several of the lime-magnesia or non-aluminous ferromagne sian minerals, namely, tremolite, pyroxene (chiefly enstatite), phlogopite mica. Chlorite re sults most frequently from the alteration of the aluminous hornblendes and pyroxenes. Epidote also results from the alteration of the aluminous varieties of hornblende and pyroxene, especially those having some lime in their com position.
Precipitations from Aqueous Solutions.— The following minerals occasionally form im portant bodies of rock. They are usually de posited from solution in water: Salt NaC1 Gnats, CaSo4.2H20 /Late or Calcite CaCOI H Silica SiOt.nlia0 ( 'to or Si:Scions Sinter) Origin of Rocks.— Two alternative hy potheses are at present recognized as possible explanations of the origin of the earth : (1) the old so-called nebular hypothesis, propounded by Kant and Swedenl)org and later elaborated by Laplace and others; (2) the newly proposed accretion theory or planetesimal hypothesis propounded by Prof. Thomas C. Chamberlin
of the University of Chicago. The first of these supposes that the earth was originally in a gaseous condition, from which, tinder its own gravity and by a radiation of its heat, it passed into a fluid state and thence to a solid form. The crusts of the earth (lithosphere) formed at the surface as the result of a cooling of the molten materials of the globe. Further more, the earth may have solidified from the centre outward as the result of pressure. Others suppose that the original crust was not only added to from below by the crystalliza tion of molten material, but also increased in thickness from above by chemical precipitations from the intensely heated hydrosphere. The accretion theory, on the other hand,. supposes that the earth as a whole never was in a gase ous or even fluid condition, but was built up by the infalling of cold, solid particles of matter called planetesimals; that the present internal teat is the result of pressure due to gravity. •Adherents of both these hypotheses agree, how ever, that the oldest known rocks, the original or primitive rocks from which all others have been derived, are of igneous origin, that is, were once in a molten condition, from which they cooled to the solid state, and in so doing formed more or less thoroughly crystalline ag gregates of different kinds of minerals.
In discussing the and descent of rocks we must, therefore, start with the more com mon igneous varieties, and show how they have furnished materials for the others.
Rocks may be classified as (1) igneous, (2) sedimentary and (3) metamorphic.