ROCKS AND SOILS 



71 



crystal specks may be discernible. Such rocks are 

 caUed felsite, or felsitic rocks, and are usually found 

 in lava flows. 



Felsife Porphyry. Any porphyry represents two 

 phases of rock formation: first, the early develop- 

 ment of larger crystals in magma; then the develop- 

 ment of smaller crystals and/or glass. Porphyries 

 might be confused with scoria or pumice whose holes 

 are filled with other rock. However, porphyries have 

 angular crystals rather than the oval or tubelike 

 materials in some scoria or pumice. 



Most igneous rocks have some scattered, larger 

 crystals. However, a porphyry generally is not rec- 

 ognized unless the larger crystals constitute about 25 

 per cent or more of the igneous rock. 



Felsite porphyry consists of visible crystals within a 

 matrix of fine-grained, invisible or almost invisible 

 crystals. This rock is found in dikes, laccoliths, 

 and sills. 



Coarse-grained Igneous Rocks. In coarse-grained 

 igneous rocks, also called granitoid rocks, the crystals 

 are large enough to be seen with the unaided eye and 

 are more or less uniform in size. Although crystals 

 reach the size of a large pea, most grains are 3 to 5 

 millimeters in diameter. Such rocks are in laccoliths, 

 stocks, and batholiths. 



Granitoid Porphyry. Like any other porphyry, 

 granitoid porphyry consists of larger crystals in a 

 matrix of finer crystals. In this instance, the finer 

 crystals are granitoid. This porphyry also tends to 

 differ from the felsite type by having more of the 

 larger crystals — actually, many larger crystals rather 

 than a few. These granitoid porphyries are found in 

 the same places as felsite porphyries and coarse- 

 grained igneous rocks. 



Large-grained Igneous Rocks. Any igneous rock 

 with crystals larger than a pea is called pegmatite. 

 Some pegmatite crystals are more than 40 feet long. 

 Therefore, these rocks must cool more slowly than 

 any other texture class. Pegmatites form in deep, 

 well-insulated places — primarily in batholiths, but 

 also in stocks. 



Fragmenfal Igneous Rocks. Fragmental igneous 

 rock is produced by vulcanism. It consists of as- 

 sorted, cemented, "fire-broken" fragments, so-called 

 pyroclastic rocks. If such rocks are mostly volcanic 

 dust or ash, they are called tufT; if mostly larger 

 fragments, they are called breccia or agglomerate. 

 The term "breccia" is applied to any kind of ce- 

 mented fragments, including sedimentary and meta- 



morphic fragments. "Agglomerate" is the specific 

 term applied to volcanic breccia. 



COMPOSITION 



Within each of the texture classes, igneous rocks 

 are further segregated on the basis of chemical com- 

 position. They are classified as acidic, intermediate, 

 or basic. Chemical composition is indicated by color. 

 Acidic rocks usually have 25 per cent or less dark 

 minerals; intermediate rocks have approximately 50 

 percent dark minerals; and basic rocks have 75 per 

 cent or more dark minerals. This color criterion is 

 used in the attempts presented below to identify the 

 kinds of igneous rocks. Also of help in identification 

 is the fact that intermediate rocks are rich in horn- 

 blende and acidic rocks are rich in pyroxene. 



Both acidic and intermediate rocks are further sub- 

 divided on the basis of the presence or absence of 

 quartz in the rock. 



IGNEOUS ROCK TYPES 



The above criteria are used to segregate the igneous 

 rocks into the main types. Although further differ- 

 entiation can be made, the classification indicated in 

 Table 5.1 is adequate for our purposes. 



JABLB 5.1 CLASSIFICATION Of IGNEOUS ROCKS 



SEDIMENTARY ROCKS 



The diflTerent kinds of sedimentary rocks are sep- 

 arated on the basis of particle size, composition, 



