90 RELATIONS OF LIMESTONES WITH SEDIMENTS. 



If we take our examples of these strata from Great Britain, it 

 may, perhaps, be found a sufficient approximation to the ratio 

 now sought, to say the mechanical are to the chemical deposits from 

 water : 



In Palaeozoic or Primary strata, . . . 20 to I 

 In Mesozoic or Secondary strata, . . . 4 to r 



In Cainozoic or Tertiary strata, . . . 10 to I 



In these comparisons regard is had to the different proportions 

 which prevail in different districts. They would be very different 

 estimates for the Tertiary series in the Isle of Wight, and that of 

 the London basin ; and for the oolites near Bath, and those near 

 Whitby. 



From this comparison it would appear that the ratio of chemico- 

 vital to mechanical strata is greatest amongst the Secondary deposits, 

 and least amongst those of the Primary periods a circumstance on 

 which depend principally the well-marked general characters of the 

 Secondary series of rocks. It should, besides, be observed, that 

 calcareous matter very finely divided exists in nearly all the sand- 

 stones and shales of that series, and sometimes so abundantly as to 

 change, locally, Lias shale into argillaceous limestone, and Calcareous 

 Grit into arenaceous limestone, or coarse oolite. In Secondary strata, 

 the great and prominent masses of limestone almost invariably 

 attract the attention and direct the classification, and thus it happens 

 that while numerous layers of clay and sand pass nearly unobserved, 

 or are merely noticed as interpolated beds, almost every calcareous 

 bed has its characteristic local name. The almost universal diffusion 

 of calcareous matter through the mechanical strata of this large 

 group, combined with the great regularity and persistence of the 

 limestones, generally suggests theoretical notions as to the cause. The 

 observer soon learns to consider the operations by which sandstones 

 and some clays were rapidly accumulated with intermitting action, 

 like the periodical floods of a river, or some less regular inundations 

 or depressions ; while the production of limestone is regarded as 

 the result of one continuous and almost uninterrupted series of 

 chemical and organic changes. This opinion, strengthened by the 

 gradations between calcareous and sandy or argillaceous laminae, and 

 by frequent alternation amongst e%en their thinnest portions, derives 

 plausible arguments from the distribution of organic remains through 

 the several strata. In some cases they teach us plainly that sand- 

 stones, even of great thickness, were the products of temporary and 

 often of very local floods, which swept down from the land the 

 remains of animals and plants then in existence, or result from cur- 

 rents of water due to tidal action or coast interference ; but, tried by 

 the same tests, the calcareous rocks appear to have been of slower and 

 more equable production, in clearer, and more tranquil, and often 

 deeper waters. This is in harmony with the present system of 

 natural operations. The pebble beaches of our actual shores and the 

 gravel and sand-banks of our shallow seas may be compared with tht> 





