GEOVBLAND FORMATION IN FELCH MOUNTAIN EANGE. 419 



Quartz occurs iu two ways — first as rounded detrital particles, and 

 secondly as grains whidi have crystallized in place. The detrital grains, 

 which are easily recognized by their form, size, and freedom from inclusions 

 of the ores, consist of single individuals, often surrounded with rims of later 

 growth. They are also usually larger than the neighboring indigenous 

 grains. While detrital quartz is not abundant and, indeed, is often entirely 

 absent from the thin sections, its occurrence is of interest as conclusively 

 establishing the sedimentary origin of the iron-bearing formation. 



The secondary quartz grains are the most abundant constituents of the 

 thin sections, and form the general background for the other minerals. 

 They always inclose separate crystals of the iron oxides, usually in great 

 abundance, and often also chlorite and little prisms of apatite. These 

 grains usually have the shape of irregular polygons bounded by straight 

 lines, frequently with reentrant angles, and adjacent grains completely inter- 

 lock. In size the secondary quartz grains range from about 0.03 to 0.4 mm. 

 in diameter. Grains of approximately the same size occur together in bands 

 or in the rounded areas to be mentioned later. 



The iron ores include both magnetite, or martite, and crystalline hem- 

 atite, the former being much the more abundant. The magnetite and martite 

 can not be distinguished in thin section, as their color in reflected light and 

 crystalline form are the same. They occur in irregular bands composed of 

 aggregates of crystals, the edges of which interlock with the adjoining and 

 inclosed areas of quartz, and show the triangular, rhombic, and square sec- 

 tions of magnetite individuals. Magnetite also occurs in isolated, irregular' 

 aggregates interlocking with the secondary quartz grains, and of similar 

 dimensions to these, but is especially abundant as single minute crystals 

 interposed in tlie grains of secondary quartz, ranging in size from such as 

 are barely recognizable under a No. 9 objective to octahedra 0.03-0.05 mm. 

 in diameter. A single quartz grain J mm. in diameter may inclose a hun- 

 dred or more such miniite individuals. Hematite is much rarer than mag- 

 netite, and seems to be found only in the secondary quartz grains or in 

 veins. In the former it occurs in separate crystalline plates, of deep red 

 color in transmitted light, under the same conditions as to number and size 

 as the magnetic crystals. Throughout some sections, and in certain bands 

 and rounded areas in other sections, it is more abundant as inclosures than 

 magnetite. Such rounded areas formed of several quartz individuals, each 



