510 



REPORT OF NATIONAL MUSEUM, 1890. 



on the other hand all the original constituents are not essential. Thus. 

 in granite, quartz and orthoclase are both original and essential, while 

 beryl and zircon or apatite, though original, are not essential. 



(2) The secondary constituents are those which result from changes 

 in a rock subsequent to its first consolidation, changes which are due in 

 great part to the chemical action of percolating water. Such are the 

 calcite, chalcedony, quartz, and zeolite deposits which form in the 

 druses and amygdaloidal cavities of traps and other rocks. 



Below is given a list of the rock-forming minerals arranged as above 

 indicated ; only the more important of these are exhibited. Although 

 these are sufficiently described as regards their chemical and crystal- 

 lographic properties in any of the mineralogies, it has been found neces- 

 sary to label them very fully in order to indicate their relations and 

 their comparative importance iu the rocks of which they form a part. 

 It is not deemed essential that the matter of these labels be reproduced 

 here, though the following is inserted as a sample : 



MAGNETITE. Magnetic Iron Ore. 



Composition : FeO+Fe 2 3 : = :: iron sesquioxide 68.97 

 per cent. ; iron protoxide 31.03 per cent. 



Crystalline System : Isometric. 



Occurs as an original constituent in many- 

 schists and granites ; in the latter usually in mi- 

 nute crystals visible only with the microscope. 

 It is almost invariably present in many igneous 

 rocks, such as diorite, diabase, and basalt, and 

 frequently occurs in immense beds, forming a 

 valuable iron ore. 



Original minerals. 



1. Quartz. 



2. The Feldspars. 



2a. Orthoclase. 

 2b. Microcline. 

 2c. Albite. 

 2d. Oligoclase. 

 2e. Andesite. 

 2f. Labradorite. 

 2g. Bytownite. 

 2h. Anorthite. 



Original minerals — Continued. 



3. The Amphiboles. 



3a. Hornblende. 

 36. Tremolite. 

 3c. Actinolite. 

 3tZ. Arvedsonite. 

 3e. Glaucophane. 

 3/. Sruaragdito. 



4. The Monoclinic Pyroxenes, 



4a. Malacolito, 

 46. Diallage, 



