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well-defined minerals * which lay claim to the family name, be- 

 sides 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 

 viicare (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 East- 

 ern and Western worlds. These rocks in time run into each 

 other through infinite gradations, just as the mica passes insen- 

 sibly 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 some- 

 where on this stem perhaps midway between quartz and lime- 

 stone 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 in- 

 organic 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 repre- 

 sentative is ordinary quartz; combined with certain metallic 

 bases, such as alumina, iron, magnesia, lime, and the alkalies 

 (potash, soda, lithia, ca?sia, 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 varie- 

 ties 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 



* Phlogopite, a magnesia mica, commonly of bronze or copper color. 

 Biotite, or black mica, a magnesia-iron mica, of dark-green or black color. 

 Lepidomelane, an iron-potash mica, of black or green color. 

 Astrophyllite, a rare titanium mica, whose powder resembles mosaic gold. 

 Muscovite, or common mica, a potash-aluminum compound of varying color, white, gray, 

 brown, green, and even violet or rose. 



LepicV.ite, or lithia mica, a mineral of pearly luster, and grayish to rose or violet color. 

 Cryophyllite, a very rare lithium mica, of greenish color. 



