52 SEQUENCE OF STRATA. 



of such reconstructed deposits in the British strata, but they may 

 generally be detected by containing fragments of the old rocks and 

 fossils which belong to more ancient periods. Thus the Portland 

 oolite of Dorsetshire includes many fossils derived from the Carboni- 

 ferous limestone ; the ISTeocomian sands of Bedfordshire and Cam- 

 bridge contain some fossils which may have been derived from the 

 destruction of the upper part of the oolitic rocks. The Eed Crag of 

 Suffolk contains fossils derived from many of the Secondary and Ter- 

 tiary strata. While in the Boulder Clay, rock fragments and fossils 

 may often be obtained from an immense variety of formations. 



Vertical Sequence of Rocks. We may carry this generalisation 

 concerning the order of horizontal deposits one step farther by remark- 

 ing that as pebble beds and sand form near to shore, and clays and 

 organic limestones farther out to sea, we have in the mineral character 

 of the deposit a rough means of discovering the general direction in 

 which land existed at the time when a geological formation was accu- 

 mulating, if we can only determine the horizontal sequence of its 

 mineral material It follows from the horizontal order of rocks that 

 there must be a similar vertical order or succession in time, because 

 land is always in process of being upheaved or depressed, and is there- 

 fore enlarged or diminished in area, so that the positions of the coasts 



Fig. I2. 1 Sequence of Deposits on a Sea-bed 



advance out to sea from their former position or retreat in the reverse 

 direction. This may be better understood from a diagram (fig. 12). 

 Here we suppose a coast of crystalline rocks (g) to be destroyed by 

 the sea, and the result is seen in the deposit of (s) sands, (c) clays, 

 and (I) limestones. Then if the land (g) is depressed so that high- 

 water mark stands at x, it follows that the point from which the 

 deposited materials are derived being farther inland (p), the sediment 

 cannot be carried by tidal movement so far out to sea. Hen< 

 a new sand will be deposited which will be continuous with tl 

 old sand (s) but since sand can only be carried a definite distain 

 from shore, it results that after the requisite depression no part 

 the new sand will rest upon the old deposit. And then a nei 

 clay will be deposited on the top of the old sand (*) ; and a new linn 

 stone on the old clay (c). If we suppose, again, the sinking dowi 

 of the land to progress a stage beyond so that the high- water marl 

 extends further inland, then as the deposits always retain the same 

 order and relative distances from the shore, it results that the sane 

 will accumulate still farther away from the area where we saw it origi- 

 nally (fig. 1 3) ; a third clay will be formed upon the second sand, and 

 third limestone upon the second clay. Here, then, is a vertical sequence 

 of sand, clay, limestone, resulting from the removal of the licav: 

 1 Seeley : Annals and Mag. Xat. Hist., December, 1867. 



