36 INVERTEBRATE PALAEONTOLOGY 



twenty feet in thickness, fossils of many kinds are so 

 crowded as to be almost in contact. In Gloucestershire, 

 where depression allowed accumulation of sediment 

 reaching two hundred feet in thickness, fossils are 

 generally more widely separated. There is no reason to 

 suppose that the abundance of life was seriously different 

 in the two districts at any time in the Inferior Oolite 

 stage ; the apparent difference is due to the amounts 

 of sediment in which the fossils are embedded. The 

 fossiliferous character of the Chalk Rock, and lower parts 

 of the Upper Chalk, in the South of England, is a 

 further example of this phenomenon. 



(IV) NODULES (PL ii. fig. 2 ; PL iv. fig. i) 



Concretionary structures are characteristic of many 

 sedimentary rocks, especially argillaceous types. Con- 

 cretions differ in composition and origin, but they agree 

 in being developed after accumulation of the sediment. 

 Calcium carbonate, ferrous carbonate and opaline Silica 

 are frequently present in clays and limestones. Their 

 ready solubility in surface water ensures removal from 

 exposed layers, while their relative insolubility in non- 

 aerated water encourages precipitation at greater depths. 

 Petrifaction often results from differential solubility, but 

 formation of concretions is perhaps more usual. In 

 some deposits, such as the Old Red Marls of Hereford- 

 shire, the Lower Liassic clays of Dorsetshire, or the 

 Upper Chalk, calcareous or siliceous ingredients have 

 segregated into apparently " sedimentary " sheets of 

 Cornstone, Cement-stone and Flint, In others, such as 

 the Coal-Measure shales, the Upper Lias of Yorkshire 

 (PL iv. fig. 3), or the London Clay, Ironstone, Cement- 

 stone or Septarian nodules are developed. Precipitation 

 of dissolved minerals is assisted by, and usually centred 

 around, some particle of solid matter with similar 



