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their general characters. No limit could there- 

 fore be drawn in an arrangement of this nature, 

 between those specimens, or rocks, in which such 

 minerals abound, and those in which they occur, 

 as it were, by chance. For these reasons, such 

 compounds, even where rendered very remark- 

 able by the predominance or abundance of some 

 particular accidental or superfluous mineral, have 

 not been enumerated in the synopses, even among 

 the lowest divisions of varieties. Micaceous schist 

 is an example in point. In this rock, a single 

 garnet only is sometimes contained in a large frag- 

 ment ; while, at others, these crystals are so nu- 

 merous as to form a large part of the bulk of the 

 rock. The practice has here been, not to consi- 

 der this as a distinct variety, but as an ordinary 

 specimen containing imbedded minerals. 



The same rule has further been extended to 

 all the cases where minerals occur in a state of 

 mixture, however considerable ; provided these 

 are not found in that list which is generally re- 

 ceived by geologists as containing all the minerals 

 essential to rocks. It may perhaps be hereafter 

 found necessary to extend that catalogue; in 



