70 



PETROLOGY AND PEDOLOGY: 



CALCITE 



Calcite is of common occurrence and barely can be 

 scratched by a penny. It is colorless, white, or has 

 pale tints of color. It is similar in appearance to 

 dolomite, but can be distinguished from it in that it 

 fizzes readily in acid without being scratched. 



MICA 



Micas are the softest of the common rock-forming 

 minerals. They often can be scratched with a finger- 

 nail. This group of minerals is of common occur- 

 rence in igneous and metamorphic rocks. They are 

 glassy or pearly minerals that are clear, white, black, 

 or colorful. Micas readily cleave by splitting into 

 paper-thin, transparent, flexible plates. Their cleav- 

 age causes an extreme mirrorlike shine. 



ROCK TYPES 



There are three major groups of rocks, igneous, 

 sedimentary, and metamorphic. Igneous rocks are made 

 by solidification of magma when it penetrates the 

 earth's crust. Magma may solidify within the crust, 

 shaping intrusive igneous rocks, or upon the surface, 

 forming extrusive igneous rocks. 



Sedimentary rocks are the erosion products of any 

 kind of rock. They are composed of fine particles 

 (marl, soil, loess, clay, sand, etc.), large particles 

 (gravel, talus, till, etc.), fine to large cemented 

 particles (shale, sandstone, conglomerate, breccia, 

 etc.), organic products (amber, coal, limestone, pe- 

 troleum, etc.), and chemical deposition (limestone, 

 chert, etc.). 



Metamorphic rocks are wrought from the altera- 

 tion of other rocks. The diflTerent appearances of 

 these rocks are dependent, at least in part, upon the 

 parent material. Metamorphism produces soapy, 

 noncrystalline rocks (soapstone and serpentine); 

 crystal masses (marble and dolomitic marble); hard, 

 glassy masses, (quartzite); separable sheets (slate); 

 thin, often separable, crystalline layers (schist); and 

 distorted, crystal masses (gneiss). 



IGNEOUS ROCKS 



The various types of igneous rock can be recog- 

 nized by their texture and composition. Texturally 



they range from glassy to fine grained to mixed fine 

 and coarse grained to coarse grained to coarse and 

 large grained to large grained. Also, any of the rocks 

 can occur as large masses or as fragments. Insofar as 

 composition is concerned, the chemicals found in the 

 molten magma determine the minerals present in the 

 solidified product, the igneous rock. If the rock has a 

 large amount of acid-forming silica, it is called acidic 

 or silicic; if it is low in silica and higher in sodium, 

 calcium, magnesium, and carbonate, it is basic. 

 There are also intermediate types. 



TEXTURE 



The texture of igneous rocks is determined by their 

 rate of cooling. When cooling is very rapid, a glassy 

 texture results. Progressively longer periods of cool- 

 ing allow larger and larger crystals, the fine-grained 

 to large-grained rocks, to be formed. This rate of 

 cooling is associated with the kind of magma flows. 

 For example, lavas cool very rapidly and form glassy 

 or fine-grained rocks. On the other hand, deep 

 batholith magma is insulated, so it loses heat very 

 slowly and grows to coarse or large-grained rocks. 



Between the extremes of glassy and large-grained 

 rocks are many texture classes. However, it should be 

 realized that there is actually a continuous gradation 

 of texture from one extreme to the other; the texture 

 classes are a matter of convenience for the purpose of 

 identification and study, and one should not expect 

 always to be able to assign any igneous rock to a 

 definite texture class. 



Glassy Igneous Rocks. Glassy igneous rocks are 

 most likely to harden from magma that explodes into 

 the air, lies upon the surface of a lava flow, or re- 

 mains on a volcano. In such places, cooling is so 

 rapid that chemical compounds do not segregate and, 

 therefore, minerals do not form. The rock is thus a 

 glass, or slag. 



Texture subtypes are dependent upon the physical 

 structure of the rocks. If the glass is massive (i.e., 

 has no holes), the rock is obsidian; if it has fine, 

 closely spaced holes, the rock is pumice (which often 

 floats in water); if it has larger and fewer, irregu- 

 larly spaced holes, the rock is scoria. The holes re- 

 sult when lava containing gas bubbles is solidified. 



Fine-grained Igneous Rocks. Fine-grained igneous 

 rocks have individual minerals in the shape of crys- 

 tals, but the crystals are so small that the rocks 

 usually appear noncrystalline. On occasion, fine 



