CALCAEEOUS PRAGMENTAL EOCKS 121 



reduced the level of the land as to greatly diminish the currents 

 and consequent carrying power of the seaward-flowing streams. 

 Such beds, on consolidation, yield then what are commonly 

 known, in the order of their formation, as conglomerates, sand- 

 stones, shales and argillites, or clay slates, the shales occu- 

 pying, both in texture and composition, a position intermediate 

 between the argillites and sandstones. 



The table on page 120 shows the varying character of the rocks 

 included under this name. Those given in columns I and II carry 

 sulphur in combination with iron, as iron pyrites (FeSa). This, 

 on decomposing, through the action of meteoric waters, yields 

 iron sesquioxides and sulphuric acid, the latter combining with 

 a portion of the alumina in the rock to form sulphate of alumi- 

 num, or common alum. Hence they have been called alum shales. 



Laterite is a red, ferruginous residual clay found in tropic 

 and semitropic regions. (See p. 298.) CatUnite, or Indian 

 pipe-stone, is an indurated clay rock formerly used by the Da- 

 kota Indians for pipe material. The name porcellanite has 

 been given to a compact porcelain-like rock consisting of clay 

 indurated by igneous agencies. The name wacke is sometimes 

 nsed to designate an earthy or compact, dark-colored clayey 

 material resulting from the decomposition in situ of basaltic 

 rocks. Adohe is the name given to a calcareous clay of a 

 general gray-brown or yellowish color, very fine grained and 

 porous, and which is widely distributed throughout the more 

 arid regions of the West. It is described in greater detail 

 under the head of Soils (p. 320). Loess is a somewhat similar 

 material forming the surface soil over wide areas in the Missis- 

 sippi valley, and at times sufficiently plastic for brick making. 

 (See also p. 315.) 



(3) The Calcareous Group. — Here are brought together a 

 small series of fragmental rocks composed mainly of calcareous 

 material, but of which the organic nature, if such it had, is not 

 apparent. These rocks form at times beautifully brecciated 

 marbles. Their structure may be best comprehended by remem- 

 bering that the original beds, whether crystalline or amorphous, 

 whether fossiliferous or originating as chemical precipitates, 

 have been crushed and shattered into fragments, and then, by 

 infiltration of lime and iron-bearing solutions, slowly cemented 

 once more into firm rock. The composition is essentially the same 

 as the ordinary sedimentary limestones and need not be further 



