14 DEFINITIONS AND LAWS OF GEOLOGY. 



forms, and this dissimilarity furnishes the facts upon which the Groups of rocks are 

 distinguished from each other. Comparison of the fossils shows a progression in 

 development along an ascending scale toward the higher and more enduring plants 

 and animals, and the extinction of lower or less highly organized forms. 



§ 21. Sanddone is a rock made of sand derived from a silicious rock. When 

 pure it is used for making glass. Iron usually colors it red or yellowish, and often 

 cements it into good building stone. When a little clay is intermixed it is called 

 freestone, and if it contains gravel it is conglomerate, or if loosely cemented in the 

 air and not under water a pudding-stone. When sandstone is subjected to heat and 

 pressure it is metamorphosed and becomes quartzite. 



Sliale is a soft, fine-grained, aluminous rock, in layers. If it is pure it is clay 

 shale; if it contains sand it is sandy shale; if bituminous matter, bituminous shale. 

 When the shale is hardened it becomes slate. Slate rocks among the metamorphic 

 series are called schists. The clay slate used in North Carolina for making slate- 

 pencils is called pyrophyllite. 



IAmestone is ordinarily composed of lime and carbonic acid, with impurities of 

 clay, sand, and iron. Hydraulic limestone contains clay and magnesia. Magnesian 

 limestone is called dolomite, after Dolomieu, a mineralogist. Lithographic stone is a 

 very even-grained, compact limestone, usually of buff or drab color. Chalk is a soft 

 limestone, and marble is a hard crystalline limestone. Gypsum, alabaster, calcite, 

 dogtooth spar and satin spar are names given to crystalline limestone. 



§ 22. The general order of superposition of the rocks of North America has 

 been ascertained, and they have been divided into Systems and Groups. Another 

 division has been made, founded on the organisms that occur in the rocks, viz : 

 Eozoic, Palaeozoic, Mesozoic and Caenozoic. Some use the word Archaean instead of 

 Eozoic. The Eozoic includes the Laurentian and Taconic Systems. The Palaeozoic 

 includes the Lower Silurian, Upper Silurian, Devonian, Subcarboniferous and Car- 

 boniferous Systems. The Mesozoic includes the Triassic, Jurassic and Cretaceous 

 Systems. The Caenozoic is synonymous with the Tertiary System. These Systems 

 may be very closely parallelized with the strata of Europe and other parts of the 

 world. The words " System*' and "formation" are in use with this nomenclature, 

 as Devonian "System" or Devonian "formation," but more generally they are both 

 omitted as unnecessary appendages to the names of the divisions. 



The Taconic is introduced in many places with conglomerate layers resting un- 

 conformably upon the Laurentian ; the Lower Silurian commences with the Potsdam 

 sandstone, the Upper Silurian with the Medina sandstone, the Devonian with the 

 Oriskany sandstone, the Subcarboniferous with the Waverly sandstone, and the Coal 

 Measures with the Carboniferous Conglomerate. Each of these great divisions 

 commences with drifted materials, and important changes of the fauna. They are 

 each capable of subdivision into Groups, and they are not only convenient in the 

 discussion of the science, but they are, to a certain extent, founded in nature. 



§ 23. For the purpose of more definite classification these larger divisions are 

 subdivided. Each subdivision is called a "Group," and it generally bears the name 

 of the place where first studied and described ; as, the Potsdam Group, so named be- 

 cause the strata were first studied and described at Potsdam, New York. This 

 method is preferred to any other, because the geographical name, when combined 

 with the word " Group," is sufficiently technical. It can not be used for any other 



