396 GEOLOGY 



of the mud, earth, clay, etc., of which the shale was made. Sand- 

 stones are sometimes divided into beds by shaly or clayey partings, 

 but quite as often by variations in the coarseness of the sand itself, 

 or by the presence of laminae or layers that are less coherent than 

 those above and below. Sandstones may be thick or thin-bedded 

 and their bedding passes insensibly into lamination. 



Sand is usually deposited in relatively shallow water where 

 it is subject to much shifting before it finds a permanent lodgment. 

 In the course of this shifting, bars or reefs 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 face opposite. The inclined Iamina3 thus formed consti- 

 tute a kind of bedding, but since its planes do not conform to the 

 general horizontal attitude of the formations as a whole, it is called 

 false or cross-bedding, or, more accurately, cross-lamination (Fig. 303). 

 The same structure 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 shale 

 or limestone. The bedding of conglomerate is due chiefly to varia- 

 tions in coarseness, and laminae or beds of sand often occur between 

 the layers of coarser material. Conglomerate is often thick-bedded, 

 and cross-bedding is common. 



Lateral gradation. When the varying nature of the agitation 

 of the sea at different depths and along the different parts of a coast 

 is considered, it will be understood that deposits of one kind may 

 grade into others horizontally. Thus a bed of conglomerate 

 gravel) may grade laterally into sandstone (sand), and this into 

 shale (mud) or limestone. It is indeed more remarkable that sedi- 

 mentary strata are as regular and persistent as they are, than that 

 they sometimes grade into one another. 



Special markings. The rhythmical action of waves gives rise 

 to undulatory lodgment, known as ripple-marks (Fig. 263). They 

 are usually not the direct product of the surface-waves, as shown 

 by the fact that they are much smaller than these waves. They 

 are sometimes made by streams and stream-like currents. I'm Im- 

 proper circumstances, ripple-marks are preserve* 1 indefinitely. 

 Ripple-marks are also made by wind (Fig. 61). They are usually 



