As already explained, the metamorphic or crystalline stratified rocks are usually older than the corresponding uncrystalline rocks; but a point of greater importance here is this: the development in the silicate rocks of crystalline characters has usually made it impossible to determine the method of their deposition, whether mechanical or chemical. In a few cases, as with the rock greensand, we know that the deposition is chemical; while it is equally certain that such common silicate rocks as gneiss, mica schist, and many others, often result from the crystallization of ordinary mechanical sediments, like sandstone and conglomerate. We classify all these rocks as of chemical origin, however, without considering the mode of their deposition, because the subsequent crystallization is itself essentially a chemical process; and that justifies us in saying that these rocks are made what they now are chiefly by the action of chemical forces. Whatever they were originally, they have become, through their crystallization, rocks having a definite mineral composition which can be classified chemically. Some of the details of the classification of this group, as shown in the table, require explanation. In studying the silicate minerals it was stated to be important to recognize two classes—the _acidic_ and the _basic_—the dividing line falling in the neighborhood of 60 per cent. of silica. This division is important simply because Nature has in a great degree kept the acidic and basic minerals separate in the rocks; and few things in lithology are more important than the distinction of the silicate rocks in which acidic minerals predominate from those in which basic minerals predominate. The amount of silica which any rock of this group contains is shown at a glance by the chart. The vertical broken lines, with the figures at the top, indicate the proportion of silica, which increases from 30 per cent. on the right to 85 per cent. on the left; so that the percentage of silica which a rock contains determines its position, the acidic species being on the left, and the basic on the right. As most of these rocks are composed of two or more minerals mixed in very various proportions, there is usually a wide range in the percentage of silica which the same species may contain; and this is expressed in each case by the length of the dotted line under the name of the rock. Thus, in syenite, the silica ranges from 55 per cent. to 65 per cent. The horizontal line in the chart separates the gneisses, containing feldspar as an essential constituent, from the schists, in which feldspar is wanting, except as an accessory constituent. We will take up the gneisses first.