THE ROCKS OF THE EARTH'S SURFACE SHELL 37 



lines like the walls of a honeycomb, and when the cracks have been 

 filled at high tide, a structure is produced which may later be 

 recognized and is usually referred to as " mud-crack " structure. 

 This structure is of special service in distinguishing marine de- 

 posits from the subaerial or continental deposits. 



A variation in the direction of winds of successive storms 

 may be responsible for the piling up of the beach sand in a pecul- 

 iar " plunge and flow " or " cross-bedded " structure, a structure 

 which is extremely common in littoral deposits, though simu- 

 lated in rocks of eolian origin. 



The order of deposition during a transgression of the sea. 

 Many shore lines of the continents are almost constantly migrat- 

 ing either landward or seaward. When the shore line advances 





FIG. 18. Diagram to show the order of the sediments laid down during a trans- 

 gression of the sea. 



over the land, the coast is sinking, and marine deposits will be 

 formed directly above what was recently the " dry land." Such 

 an invasion of the land by the sea, due to a subsidence of the coast, 

 is called a transgression of the sea, or simply a transgression. 

 Though at any moment the littoral, shoal water, and offshore 

 deposits are each being laid down in a particular zone, it is evi- 

 dent that each must advance in turn in the direction of the shore 

 and so be deposited above the zones nearer shore. Thus there 

 comes to be a definite series of continuous beds, one above the other, 

 provided only that the process is continued (Fig. 18). At the 

 very bottom of this series there will usually be found a thin bed 

 of pebbly beach materials, which later will harden into the so- 

 called basal conglomerate. If the size of the pebbles is such as to 

 make possible an identification, it will generally be found that these 

 represent the ruins of the rock over which the sea has advanced 

 upon the land. 



Next in order above the basal conglomerate, will follow the 

 coarser and then the finer sands, upon which in turn will be laid 

 down the offshore sediments the muds and the lime deposits. 



