2 sedimentary

2 Description of Sedimentary RocksIn the case of sedimentary rocks we can start with a broad two-fold classification. The first group consists of those rocks in which the component materials have been derived from preexisting rocks, and have been moved-perhaps great distances-to the ultimate site of deposition①. Rocks of this type are commonly referred to as clastic or detrital. A second broad group of sedimentary rocks consists of those rocks whose component materials have been formed either in or very close to the ultimate site of deposition. Such rocks include chemical precipitates and certain types of sediments which consist largely of the broken shells of organisms that lived in or near the depositional environment. This second group of rocks includes the limestones and dolomites and is commonly referred to as chemical.Clastic rocks are generally subdivided largely on the basis of the grain size of the component material. The commonly used terms are: CONGLOMERATES AND BRECCIAS. These are rocks which consist of pebble-and cobble-sized material. In a conglomerate the grains are reasonably smooth and rounded. In a breccia the grains are irregular and angular. Conglomeratesand breccias generally develop in areas quite close to their source rocks.SANDSTONES. Sediments consisting primarily of sand-sized materials constitute approximately one-fourth of the sedimentary rocks exposed on the continents. The predominant minerals of sandstones are quartz and feldspars, and in many sandstones quartz constitutes virtually the entire sediment. Minerals such as biotite, hornblende, and augite, which are common in igneous source rocks, are comparatively unstable in contact with the air and water of the earth's surface. Hence the proportion of quartz and feldspar is increased in sedimentary rocks over its percentage in igneous rocks. Geologists commonly subdivide sandstones into three different groups: orthoquartzites, arkoses and graywackes.SILTSTONES. These are rocks that are composed of hardened silt.SHALE. Shale is best defined as consisting of extremely fine-grained minerals in the size range designated as clay. In addition to this size definition for shale particles, many of the minerals that are found in the shale are commonly referred to as clay minerals. In fact, fine-grained muscovite is one of the major clay minerals and is probably the most common component ofthe ordinary shale. Nearly two-thirds of all sedimentary rocks are shale.The chemical rocks are primarily limestones, consisting of the mineral calcite and dolomite. Carbonate rocks comprise approximately five percent of all of the known sediments. Limestones are classified as chemical largely by a terminological convention, and because it②is felt that the material constituting limestones has generally been deposited by some type of chemical precipitation from water.Apparently almost all limestones were originally deposited in the ocean. One of the best pieces of evidence for a marine origin for the average limestone is the fact that the bulk of the calcite in most limestones consists of small pieces of shells of marine invertebrates③. Many limestones, in fact, consist almost exclusively of pieces of brachiopods, mollusks, corals, or other organisms. In addition to shell fragments, limestones contain a large variety of other types of material. Oolites, for example, are small spherical aggregates of calcium carbonate.Dolomites are less common than limestones, although they are extremely abundant in deposits of certain periods. There appear to be two major types of dolomites. The first occurs as broad extensive beds of relatively finegrained homogeneousmaterial. It looks almost identical to fine-grained limestone and can be distinguished only by simple chemical tests. The other type of dolomite consists generally of more coarsely crystalline material. This type of dolomite generally does not form extensive beds but may occur as irregular patches within a sequence of limestones. In many cases these patches cut across the bedding or other original features of the limestones, and the dolomite quite clearly has formed after deposition of limestone.Halite, gypsum, and related materials constitute a very small portion of the geologic record and occur in only a few places on the earth's surface. These minerals are generally grouped together under the term evaporites.Two other sediments of extremely restricted occurrence should also be mentioned. One of these is a rock which consists exclusively of fine-grained SiO2and is called chert. In most cases, this SiO2 is not in the crystalline form of quartz, and in some rocks the material is completely amorphous. Another minor but highly important, type of sediment is referred to as iron formation. Iron formation generally consists of iron oxides, such as magnetite and hematite, interbedded with chert. The importance of these rocks is that a small amount of chemical alteration converts them into highly concentrated hematite rocks,generally by a partial removal of the chert, and the resulting hematite rock forms the major iron ore deposits of the world.New Wordsaggregate [ ✌grigeit] n. 集合体amorphous [☜m :f☜s] a. 非晶质的arkose [ a:k☜us] n. 长石砂岩augite [ :d✞ait] n. 辉石biotite [ bai☜tait] n. 黑云母breccia [ bret☞☜] n. 角砾岩broad [br :d] a. 概括的chert [t☞☜:t] n. 燧石coarsely [ k :sli] ad. 粗, 粗糙地cobble [k bl] n. 中砾石convention [k☜n ven☞☜n] n. 惯例definitely [ definitli] ad. 确定地description [dis krip☞☜n] n. 描述detrital [di trait☜l] a. 碎屑的evaporite [i v✌p☜rait] n. 蒸发岩extensive [iks tensiv] a. 广阔的group [♈ru:p] n. 群，类，分类hardened [ ha:d☜nd] a. 变硬的，固结的hematite [ hem☜tait] n.赤铁矿hornblende[ h :nblend] n. 角闪石mollusk [ m l☜sk] n. 软体动物muscovite [ m✈sk☜vait] n. 白云母oolite [ ☜u☜lait] n. 鲕石partial [ pa:☞☜l] a. 部分的patch [p✌t☞] n. 碎片,团块pebble [pebl] n. 卵石predominant [pri d min☜nt] a. 主要的，占优势的sand-sized 砂粒级的terminological [ֽt☜:min☜l d✞ik☜l] a. 术语学的two-fold 二倍的，二重的。