STRUCTURE AND POSITION. 



171 



-Sections of horizontal and inclined strata: 

 ss, sandstone; sh, shale: Ls, limestone. 



again into still smaller lamina?. These terms are purely relative, and 

 are therefore somewhat loosely used. Usually, however, the term 

 stratum refers to the mineralogical character ; the term layer to sub- 

 divisions of a stratum distinguishable by difference of color or fine- 

 ness ; and the term lamina to 

 those smallest subdivisions, evi- 

 dently produced by the sorting 

 power of water. For instance, in 

 the annexed figure we have three 

 strata of sandstone, clay, and 

 limestone, each divisible into two 

 layers diif ering in fineness or com- 

 pactness of the material, and all 

 finely laminated by the sorting 

 power of water. The lamination, 

 however, is not represented, ex- Fig. ias 

 cept in the clay stratum, sh. 

 There is another structure represented in the figure — viz., the cross 

 fractures or joints. These, however, are not peculiar to stratified rocks, 

 and will be discussed at another time. 



Extent and Thickness. — Probably nine-tenths of the surface of the 

 land, and, of course, the whole of the sea-bottom, are covered with 

 stratified rocks. Even where these are wanting it is because they 

 have been removed by erosion or else covered up and concealed by 

 fused matter outpoured on the surface. This proves that every portion 

 of the surface of the earth has been at some time covered with water. 

 The extreme thickness of stratified rocks is certainly not less than 

 twenty miles ; the average thickness is probably several miles. 



Kinds of Stratified Rocks. — Stratified rocks are of three kinds, and 

 their mixtures, viz., arenaceous or sand rocks, argillaceous or clay 

 rocks, and calcareous or lime rocks. Arenaceous rocks, in their inco- 

 herent state, are sand, gravel, shingle, rubble, etc., and in their com- 

 pacted state are sandstones, gritstones, conglomerates, and breccias. 

 Conglomerates are composed of rounded pebbles, and breccias of angu- 

 lar fragments cemented together. Argillaceous rocks, in their inco- 

 herent state, are muds and clays; partially consolidated and finely 

 laminated they form shales, and thoroughly consolidated they form 

 slates. Calcareous rocks are chalk, limestone, and marble. They are 

 seldom in an incoherent state, except as chalk. 



These different kinds of rocks graduate into each other through 

 intermediate shades. Thus we may have argillaceous sandstones, cal- 

 careous sandstones, and calcareous shales or marls. 



The most important points connected with stratified rocks we will 

 now, for the sake of greater clearness, bring out in the form of distinct 



