1028 Subsurface Geologic Methods 



Compositionally banded or layered igneous rocks (gneisses, extrus- 

 ives, etc.) are represented, relatively, on the field sheet by compositional, 

 colored bands. This layering is similarly indicated on office maps, except 

 that phanerites are represented by appropriately colored bands of dash 

 symbols indicating compositional banding. 



Pegmatites, aplites (micropegmatites), and graphic textures are in- 

 dicated by modified symbols of relative size, 



Hydrothermal effects are represented by a chromatic sequence rang- 

 ing from yellow to red, and then to brilliant purple. The legend is adapt- 

 able to local conditions by substituting other alteration minerals and effects 

 in place of those indicated on the chart. 



Alteration representation is quantitatively proportional to amount and 

 degree of hydrothermal effect, and is shown by color overlay (blanket 

 alteration), stipple (disseminated alteration), or lines (joint or fissure 

 alteration). Gradational effects are indicated by corresponding grada- 

 tional-color intensity. Two or more colors may be applied in superposition 

 to indicate two or more alteration effects. On office maps, an artist's air 

 brush may be used to apply coarse, alteration-color stipple in superposition 

 indicating alteration effects singularly and collectively. 



Vein quartz and other gangue minerals are shown in red, the inten- 

 sity of red indicating degree of replacement. The intensity of red is in- 

 creased by superimposed, black (6H) pencil lines. Silicification is in- 

 dicated by lightly applied red. 



Metallization (sulphides, oxides, etc.) is indicated pictorially by 

 brilliant purple stipple (disseminations) or scaled lines (veins). Small 

 veinlets (less than 3" wide), stringers, and mineralized zones are shown 

 by generalized representation (small purple lines of proper attitude) ; the 

 amount of colored area is proportional to the amount of mineral. 



Supplementary, descriptive side notes indicate width and type of min- 

 eralization. Most minerals are listed by chemical formula: 



Qtz+ + +,FeS2++,PbS,ZnS,CaC03~,CuFeSo=, etc. An arrow indi- 



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cates observed paragenetic sequence; e.g., sphalerite is later than galena. 



Plus and minus signs show relative amount of each mineral. 



Deformational effects such as faults are shown in blue. The inten- 

 sity of blue indicates the fault-clay fineness, and the consequent relative 

 amount of displacement. Gritty fault clays are indicated by pale blue; 

 sticky, gummy clays by dark blue (derived by retracing fault lines with a 

 sharp 6H pencil). Fault clays and breccias (3" and up) are plotted to 

 scale on 50' scale maps. Shear zones are represented by faint blue lines of 

 either regular or irregular orientation conforming to observed attitude. 

 Breccia is indicated by blue background with superimposed stippled colors 

 representing breccia fragments. 



Unconsolidated material such as soil is mapped. The map must be a 

 recording of observed facts to be of value in mineral exploration; it must 

 be possible to reinterpret data at any time. All area traversed should be 



