CLAY AND ITS MODE OF OCCURRENCE. 9 



of the deposit we can read much regarding the conditions under 

 which it was formed. If, therefore, in the same bank alternating 

 layers of sand, clay and gravel are found, it indicates a change 

 from disturbed to quiet water, and still later rapid currents over 

 the spot in which these materials were deposited. The com- 

 monest evidence of current deposition is seen in the cross-bedded 

 structure of some sand beds, where the layers dip in many differ- 

 ent directions, due to shifting currents, which have deposited 

 the sand in inclined layers (PI. I, Fig. 2). The beds of thinly 

 stratified or laminated sands and clays, found in many cases over 

 the Woodbridge fire-clay bed, are another example of rapid 

 changes in the conditions of deposition. 



Sedimentary clays can be distinguished from residual clays 

 chiefly by their stratification, and also by the fact that they com- 



netware ^^-.T^r-jr ^= _ . " : — £-;j -. -" — — , »- - 



\n4y clay ~.'..— .'• 'rr, :'.~? : rz~'. >T ? — ■ •^-'■'; . ; TT~. .— *-. *. .* ' * """■, ; ~ ~ 



Bed Rock 



Fig. 2. 



Generalized section showing how beds of clay may vary horizontally and vertically. 



monly bear no- direct relation to the underlying rock on which 

 they may rest. 



All sedimentary clays resemble each other in being stratified, 

 but aside from this they may show marked irregularities in 

 structure. 



Thus, any one bed, if followed from point to. point, may show 

 variations in thickness, pinching or narrowing in one place and 

 thickening or swelling in others, as shown in Fig. 2. 



In digging clay the miner often finds streaks of sand extend- 

 ing through the deposit and cutting through several different 

 layers, these having been caused by the filling of channels cut in the 

 clay deposits by streams after the elevation of the former to dry 



