136 



THE YOUNG NATURALIST. 



In the north-west- the middle beds have thinned out very considerably, but 

 the three-fold division is maintained throughout. 



The Triassic system has been so named because of its three great divisions 

 — Keuper, Muschelkalk, and Bunter ; the middle division is calcareous and 

 though absent in Britain, it is well developed on the continent. The same 

 three-fold arrangement may be seen in the Jurrassic system, the limestones 

 of the Lias and the Oolite alternating with clay shales and sandstones. 



In the Cretaceous formations we find the chalk marl and the Gault clays 

 underlying the chalk ; while in the London basin we see the Thanet sands 

 conformably overlying the chalk, and although these sands belong to the low- 

 est of the Tertiary formations, still as it lies conformably upon the chalk, and 

 completes the three-fold series of argillaceous, calcareous, and arenaceous 

 strata in the usual order of superposition, we are quite justified in taking it 

 as an illustration. 



Throughout the Tertiary group this three-fold arrangement of strata cannot 

 be so clearly traced, the areas of upheaval and depression are more limited, 

 the changes were not so great, and the movements seem to have taken place 

 in a more irregular and spasmodic manner, hence the results are not so mani- 

 fest as they are in most of the older formations already mentioned. 



This three-fold division clearly indicates three periods in the history of the 

 earth : — the first, a period of depression ; the second, a period of quiescence ; 

 and the third, a period of upheaval : or 1, movement; 2, rest; 3, movement. 



We have pointed out that the lower strata are more generally argillaceous, 

 this is especially the case in the Oolite and the Cretaceous formations ; this 

 is probably due to the wearing down of alluvial shores and cliffs, as the land 

 slowly subsided beenath the waters of the ocean. 



Most of the organisms that produce limestone can only exist in deep, clear 

 water, which is free from mud or sand, and hence we never find limestone as 

 a primary deposit in any formation, it generally occurs near the middle, or 

 during the time of quiescence. 



Arenaceous deposits are generally produced by the degradation of granite 

 or other plutonic rocks, and also the denudation of indurated grits and sand- 

 stones, such as would be likely to come to the surface after a period of great 

 and extensive upheaval. 



May we not thus read in the mineral character, and in the natural arrange- 

 ment of sedimentary rocks, apart from the fossils they contain, the history of 

 the geological changes which have taken place in the earth's crust. 

 Macclesfield, May, 1885. 



