74 



PETROLOGY AND PEDOLOGY: 



ditions are from deep within the earth's crust. The 

 temperature is derived from the one degree Faren- 

 heit increase of temperature for every 60 feet of 

 depth and the pressure from the mass of materials 

 above the site of metamorphism. 



There are definite grades of metamorphism, and a 

 graded series of metamorphic rocks can stem from a 

 single kind of rock. Such a series is in direct re- 

 lationship to the intensity of metamorphism. For 

 example, progressive metamorphism will alter shale 

 to slate, slate to phyllite, phyllite to schist, and 

 perhaps schist to gneiss. This tends to be an over- 

 simplification, because one type of schist may give 

 rise to another schist. Another example of progres- 

 sive metamorphism would be peat to lignite, lignite 

 to bituminous coal, bituminous to anthracite coal, 

 and anthracite to graphite. Therefore, metamor- 

 phism is a continuous process and in naming 

 metamorphic rocks — or any other rocks, for that mat- 

 ter — man is indicating "landmarks" in a continuous 

 series. 



CLASSIFICATION 



Metamorphic rock classification is based upon 

 structure and texture, factors used to designate 

 foliate and nonfoliate categories. The foliate, or 

 leaflike, group consists mainly of thin, flat minerals, 

 usually mica or chlorite, that cleave (separate along 

 structural lines of the mineral) parallel to the flat 

 dimension. Therefore, these rocks have a tendency to 

 split into flat sheets along the plane of leaflike min- 

 eral orientation which also is the plane of cleavage. 

 The nonfoliate types are neither leaflike minerals 

 nor do they separate easily into flat sheets. Rather, 

 they usually occur as crystalline masses similar to 

 the coarse-grained igneous rocks, or as microcrystal- 

 line masses (although some crystals may be visible) 

 similar to fine-grained igneous rocks. 



Foliate Meiamorphic Rocks. In the discussion of 

 progressive metamorphism the main kinds of foliate 

 rocks, slate, phyllite, schist, and gneiss, were named. 

 Although, there are many variations of each kind, only 

 the main types are mentioned. Slate can be of almost 

 any color. It originates and grades upward from 

 shale and merges into phyllite. However, slate 

 usually is a fine-grained, homogeneous rock that sep- 

 arates into relatively smooth, thin to thick sheets. 

 Well-formed, thin slate is used as blackboards. Slate 

 might be confused with cherty shale; however, the 



latter is much harder (cherty shale cannot be 

 scratched with a knife; slate can). 



Phyllite is very much like slate, but phyllite tex- 

 ture is coarser and its composition is higher in leaf- 

 like, lustrous minerals. Although phyllite may be 

 distinguishable from other rocks, the more slate-like 

 examples often are grouped with slate and the more 

 schist-like examples with schist. 



The many kinds of schist are recognized on the 

 basis of leaflike minerals being arranged in parallel 

 lines or bands that can be separated. Large crystals 

 are uncommon; if they occur, they are scattered 

 within the primary mass of leaflike and layered 

 minerals. 



Gneisses resemble schists in that both are banded 

 and show foliation. However, only gneisses are 

 masses of coarse-grained, although often warped 

 and flattened, minerals. Moreover, gneisses are 

 rarely separable into flat slabs. They are often 

 characterized by the presence of feldspars. 



Nonfoliate Metamorphic Rocks. The main non- 

 foliate types are quartzite, marble, soapstone, and 

 serpentine. Qimrtzite is metamorphosed sandstone 

 and consists of quartz sand grains fused in quartz. 

 Therefore, when these rocks are broken and splinters 

 or curved chips are removed, breaks are through, not 

 around, the individual sand grains. .Marble is 

 metamorphosed limestone. It consists of a mass of 

 coarse crystals that will fizz in acid. Another type 

 of marble, dolomitic marble, is the metamorphic prod- 

 uct of the mineral dolomite. Although both marbles 

 assume the same colors and general form, dolomitic 

 marble will fizz in acid only after being powdered. 

 Soapstone, or steatite, is a hydrothermal alteration 

 product composed mainly of the mineral, talc. Serpen- 

 tine is formed in much the same manner as soapstone 

 and is quite similar to that rock. Both soapstone 

 and serpentine feel soapy or greasy and often are of 

 greenish tint; soapstone can be scratched with a 

 fingernail and serpentine cannot. 



METAMORPHIIC ROCK TYPES 



Table 5.3 summarizes the common metamorphic 

 rocks and their origin. 



ROCK FORMS AND STRUCTURES 



The physical characteristics of each of the three 

 major rock types are such that different kinds of 



