Page:Popular Science Monthly Volume 41.djvu/672

654 well-defined minerals which lay claim to the family name, besides an extensive list of relatives which have been formed by alteration on exposure to air and water. The series runs from the compact, glistening mica found in granite and gneiss, through many gradations of hydrous micas, until we reach the ordinary soapstones and clays. But the name properly stops when the mineral loses its glistening surfaces, for then the Latin word micare (to shine) no longer applies. Our German friends call it Glimmer, a name whose significance is readily seen.

The importance of the mica family, however, does not depend alone upon its many varieties and numerous relatives. The micas are an essential ingredient in many of our most wide-spread rocks—such as the granites, gneisses, mica schists, and their relatives, which form the continental backbone in both the Eastern and Western worlds. These rocks in time run into each other through infinite gradations, just as the mica passes insensibly into the soapstone, so that we can nowhere find hard and fast lines in the mineral any more than we can in the biological world. If we wish, then, to think of mica correctly, we must picture to ourselves a long stem with many branches, and somewhere on this stem—perhaps midway between quartz and limestone—a group of closely related minerals of peculiar scale-like structure and glistening surfaces. We must think of minerals as momentary crystallizations in an ever-changing current of inorganic matter, and not at all as fixed and final forms.

When we submit the micas to chemical analysis, we find that they all contain a large amount of silica—whose common representative is ordinary quartz;—combined with certain metallic bases, such as alumina, iron, magnesia, lime, and the alkalies (potash, soda, lithia, cæsia, and rubidia). No one mica contains all these—though there is some truth in the statement that the micas are silicates of almost everything—but the different varieties depend upon the nature and proportion of the metallic bases which combine with the silica. Thus, while common mica is in the main a silicate of potash and alumina, it also contains small quantities of other metals, such as sodium, magnesium, and iron.

There is a partial parallel here between the mineral and the organic world. Silica is, so far as we know, a compound made up