OBlG-m OF CRYSTALLINE ROCKS. 



SI 



constitute an important group, including the principal non-magnesian micas, muscovite, 

 margarodite, eupliyllite, damourite or sericite, and paragonite, excluding the rarer lepidolite 

 of veinstones, which is more highly alkaliferous. In the following list, the formulas for the 

 last four species named have been taken from Dana's " System of Mineralogy," while the 

 three given for different varieties of muscovite have been devised so as to facilitate com- 

 parison with the latter, and at the same time to represent, as near as may be, the variable 

 composition of the anhydrous mica. 



NON-MAGNESIAN OR MUSCOVITIC MICAS. 



Muscovite (a) . 

 Muscovite (b). 

 Muscovite (c) . 

 Margarodite . . 

 Eupliyllite . . . 



Damourite 



Paraaonite 



E 



Si : H 



§ 103. The frequent occurrence of muscovite in endogenous granitic veins with orthoclase 

 and albite, shows that this species, like the feldspars, may be crystallized from solutions. At 

 the same time, their composition and their geological relations suggest that this and the 

 related micas have more generally been derived, directly or indirectly, from the subaërial 

 decay of the feldspar of granitic rocks. While these micas are rare, or altogether absent 

 from the oldest granitoid gneisses, they become comparatively abundant in the yotinger 

 gneisses and their associated mica-schists and, finally, in the forms of damourite, sericite, 

 and paragonite-schists, characterize great masses of strata among the still younger transition 

 strata. "We have called attention to the fact that decayed feldspars, already changed 

 to the form of clay, and approaching to the kaolin-ratio, in which al : Si = 3 : 4, still 

 retain, in many cases, a few hundredths of alkali ; while the three anhydrous silicates 

 of alumina, andalusite, fibrolite and cyanite, which are frequently found crystallized in 

 certain mica-schists, have each the ratio, 3:2. It will be readily seen that the separation 

 of these highly alviminous silicates from clays still holding a little alkali would leave 

 residues having essentially the composition of the micas given in the above table. There 

 are, however, other mica-schists which are not accompanied by such anhydrous alumin- 

 ous silicates, but on the contrary are associated with serpentines and chloritic minerals, 

 indicating in the waters of the time a very different condition from that which we 

 have first supposed, and pointing to the intervention of soluble silicates. That these, by 

 their union with the kaolin from decayed feldspars, might yield muscovitic or acidic micas, 

 will be evident, when we note that the elements of one equivalent of kaolinite united 

 with one of thomsonite, or of natrolite, would give essentially the oxygen-ratio of mus- 

 covite or margarodite, and two of kaolinite with one of thomsonite that of damoitrite or 

 paragonite. 



§ 104. There exists another important class of hydrous alkaline aluminous silicates, 

 related to these micas in composition, but differing widely from them in structure and 



