1026 Subsurface Geologic Methods 



cated by symbols. A modified dash symbol represents pegmatitic and 

 graphic textures. 



Since colored pencil permits less detail than pen and ink office repre- 

 sentation, it is not always possible to represent symbolically texture on 

 the field-note sheet. Small igneous rock masses (narrow dikes, gneissic 

 rocks, lit-par-lit, etc.) are represented on the field-note sheet in solid color 

 with supplementary textural descriptive side notes. These rocks are then 

 graphically represented in detail on the office maps by textural symbols. 



Composition — Compositional representation of non-tectonic rocks is 

 shown by black lithic-symbol patterns; tectonic compositional effects 

 (magmatic, metamorphic, hydrothermal, and related deformational ef- 

 fects) are represented by color, as indicated in figure 541. Sedimentary- 

 rock composition is indicated by lithic symbol. The black lithic-symbol 

 patterns are superimposed and proportionately combined to show com- 

 posite compositional facies. 



Metamorphic rocks are represented by color (to show metamorphic, 

 magmatic, or hydrothermal effects) superimposed on black sedimentary 

 lithic symbols. Thus, marble is represented by a dull ultramarine super- 

 imposed on black, a limestone symbol; quartzite is represented by red- 

 brown superimposed on black sandstone dots or stipple. Micaceous bands 

 are represented as black lines because in superimposition on other colors, 

 fine structural detail is possible and black is compositionally neutral; it 

 is difficult to determine mica-schist genesis in the field because schists are 

 derived from either sedimentary or igneous rocks. The "crystalline" 

 metamorphics or gneisses are represented in the same manner as igneous 

 rocks; in the field no attempt is made to distinguish primary-igneous 

 from secondary-metamorphic gneiss. Interlayered micaceous-banded gneiss 

 is represented by black lines (quantitatively and pictorially indicating 

 amount, attitude, irregularity, and crinulation of schistose material) super- 

 imposed on the corresponding igneous compositional color. Gneisses of 

 phaneritic texture are indicated by structurally oriented compositional 

 color banding (see "Igneous rocks" which follows) . 



Magmatic effects (igneous rocks, hydrothermal alteration, veins) are 

 represented in color conforming to a compositional chromatic progression. 

 An orderly, systematic, compositional color scheme permits continuous 

 recording (rather than time-consuming, incomplete "spot" side-note re- 

 cording) of variable composition without previous lithic-mapping, unit- 

 reconnaissance determination for use in any given area. Equivocal com- 

 positional values are indicated by superimposed, chromatic, compositional 

 representation utilizing possible compositional color extremes. 



Igneous rocks are represented in chromatic progression ranging from 

 ultramarine (basic parent rocks) through various shades of green (inter- 

 mediate rocks) to yellow green (acid rocks). Alteration is shown in 

 chromatic progression ranging from^ellow (early pneumatolytic) through 

 orange (intermediate) to red (late hydrothermal). Metallization color is 

 brilliant purple, a resultant of red and blue. 



