VERTICAL SEQUENCE OF STRATA. 



53 



sediments to a greater distance in consequence of the continuous 

 depression and recession of the land from which they were derived. 



Fig. 13. Deposits formed when the shore is sinking. 



And whenever sediments succeed each other from below upward in 

 this order, it must be regarded as an evidence that the shore-line was 

 receding from the area during the whole period of time which they 

 represent. If, on the other hand, the land which is exposed to 

 destruction by the ocean were to be uplifted, so that the place whence 

 the deposited material originated would go farther out to sea, by denu- 

 dation of the shore, and the sands near shore, sand would be carried 

 farther out to sea so as to be spread over clay, and similarly clay 



Fig. 14. Deposits formed when the shore is rising. 



would be spread over the previously formed limestone. If, then, the 

 vertical sequence of rocks, limestone, clay, sand, is met with, it may 

 be regarded as evidence that during the whole of the geological time 

 which is represented by those deposits, the land was in process of 

 being upheaved, so that the shore-line was approaching the place 

 where the deposits were forming and are now seen in section in that 

 order. Therefore, since there are necessary limits over which a 

 formation preserves the same mineral character, it cannot be identi- 

 fied over very wide areas by this means. But strata can be traced 

 to distant regions by using this kind of evidence to discover the 

 physical conditions which limited, determined, and changed their 

 mineral characters, and influenced the distribution of life over the 

 geographical areas which they occupy. Hereafter we shall see how 

 these principles are practically applied. 



)If a formation consists mainly or even largely of sands, we may 

 ect to find evidences that it was deposited in shallow water, 

 and possibly near to shore. Of this we have familiar examples in 

 the ripple-marked sandstones, footprints, sun-cracks, and such like 

 phenomena, which characterise the New Red Sandstone of Cheshire, 

 and parts of the Hastings Sands in Sussex. In the clays we rarely 

 observe any indications of shallow-water conditions ; and though 

 land animals and plant remains are often found in clays, these 

 rather indicate the influx of rivers into the ocean than relative 

 nearness to land. Hence it may be inferred that if a clay is super- 

 imposed upon a sand, we are entitled to conclude that the coast, which 

 was the source from which the material was derived which accumur 

 lated on the sea-bed to form the sandstone below, became de- 

 pressed in the succeeding age, so that the source of the deposited 

 material was removed farther away, and though sands would have 

 continued to be formed, they were deposited at a distance so far off 

 that the only material which became in the British area super- 

 imposed on the sand was the finer flocculent substance which forms 



