6 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



recognized or even wrongly interpreted. Broader climatic 

 changes have affected deposits so markedly that the corre- 

 sponding conditions have doubtless been rightly adduced, 

 but the effects of the lesser changes are only now being 

 recognized. 



Sedimentation, — Certain fundamental considerations in 

 regard to the formation of sedimentary rocks first, perhaps, 

 deserve emphasis. 



As an ideal case, let us imagine that a continental area 

 composed of an igneous rock, such as granite, is subjected to 

 denudation, the gradient of the rivers being sufficient to 

 transport the bulk of the resulting detritus to the seashore. 

 As the transporting power of the rivers is checked on their 

 entering the sea, pebbles and gravelly detritus formerly rolled 

 along their beds will be deposited. Along-shore and tidal 

 currents distribute the material right or left, and storms 

 pile it up as beaches, where it often becomes mixed with other 

 coarse material resulting from the direct erosion of the land 

 by the sea. The pomiding of the waves completes the rounding 

 of the constituents already begun by the rivers, and a belt of 

 shingle extending from above high- water mark to varying 

 depths, often below low- water mark, is the result. 



Beyond the shingle belt, the river carries its sand, which 

 in turn settles dowm when the velocity of the stream falls to a 

 few millimetres per second. In due time the sand is distributed 

 as a seaward belt fringing and int er digit at ing with the shingle. 

 Similarly a band of silt is deposited, and lastly, the finest portion 

 of the burden, the mud, sinks to the bottom. It was formerly 

 considered that the last constituent, the mud or clayey material, 

 was laid down only when the w T ater w T as practically still, and 

 that its deposition therefore indicated fairly deep water w T ell 

 below the influence of waves or shore currents. On the 

 contrary, deposition of mud, resulting as it does from the 

 flocculation of the river-borne clay particles by the dissolved 



