STRUCTURAL GEOLOGY. 487 



sions of the same sort. The term stratum is sometimes apphed to one 

 layer and sometimes to all the consecutive layers of the same sort of 

 rock. For the latter meaning the term formation is often used. Some- 

 times bedding seems to have been determined by strong currents which 

 temporarily not only prevented deposition over a given area, but even cut 

 away the loose surface of deposits already made, giving a firm surface 

 from which succeeding deposits are distinct. This sequence of events 

 is sometimes shown by the truncation of laminae, and by other signs 

 of erosion. The commoner sorts of bedded rock are limestones, shales, 

 sandstones, and conglomerates. 



The bedding of liinestones is often caused by the introduction of thin 

 films of clayey material which interrupt the continuity of the hme accu- 

 mulation and cause natural partings. Sometimes, however, bedding 

 arises from variations in the physical condition of the Hme sediment itself. 

 Lamination is not usually conspicuous in pure hmestone, though it may 

 be well developed in the shaly phases of this rock. Shales are normally 

 laminated as well a^ bedded, and the lamination is often more notable 

 than the thicker bedding. Bedding in shale may arise from the intro- 

 duction of sandy lamina^, or by notable changes in the texture of the 

 shale material. Similarly, sandstones are sometimes divided into beds 

 by shaly (clayey) partings, but more often by variations in the coarse- 

 ness of the sand itself, or by the presence of laminse that are less coherent 

 than those above and below. Sometimes the layers appear to be deter- 

 mined by the compacting of the surface of sand already accumulated 

 before it was buried by later deposits. Sandstones may be thick- or 

 thin-bedded, and their bedding passes insensibly into lamination. 



Sand deposits usually take place in relatively shallow water, and 

 the sand is subjected to much shifting before it finds a permanent lodg- 

 ment. In the course of this shifting, bars are formed which usually 

 have a rather steep face in the direction in which they are being shifted. 

 The sand carried over the top of the bar finds lodgment on the sloping 

 terrace face. The inclined laminse thus formed constitute a kind of 

 bedding, but since its planes do not conform to the general horizontal 

 attitude of the formation as a whole, it is called false- or cross-hedding 

 or, more accurately, cross-lamination (see Fig. 368). The same struc- 

 ture is developed on delta fronts and generally in water shallow enough 

 to be subject to frequent agitation at the bottom. Sandstone is cross- 

 bedded more commonly than other sorts of sedimentary rock. 



