SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 1 1 



abundance of new sediment. If the elevation be gentle, the 

 increase of sedimentation will be gradual, and the phenomena 

 of penecontemporaneous erosion and seaward thickening will 

 be more evident. 



The sequence of sediments considered above is an ideal one 

 and can arise only where gravelly, sandy, and clayey detritus 

 together with dissolved material is brought down from the 

 land. At the present day extensive areas of igneous rocks 

 form but a small portion of the land-masses of continents, and 

 the greater part of the continental margins is composed of 

 ancient sediments which have, in the normal course of events, 

 been differentiated more or less effectively into the lithological 

 series discussed above. Offshore deposits derived from such 

 sediments will necessarily vary according to the constitution 

 of their parent material. Clay may thus form the littoral 

 belt entirely, or it may even lie on the landward side of a 

 sand-belt. It is probable that the character of the local 

 land-mass determines more than any other factor the distribu- 

 tion of the sediments in the offshore belt, or even of those 

 beyond it. 



If the above theoretical considerations may be adopted, 

 we should expect to find some evidence, among the extra- 

 ordinary variety of rocks in Britain, of such occurrences in the 

 geological past. For obvious reasons, examples may not be 

 numerous — evidence may not be preserved, or investigation 

 may not yet have been carried out in sufficient detail. Attention 

 may, however, be drawn to certain notable cases. 



The crossing of the time-planes by the lithological planes 

 is well illustrated by the series of fossiliferous sands occupying 

 the uppermost portion of the Lias and the lowermost of the 

 Inferior Oolite in the West of England. The sands extend 

 from the Dorset coast to the Cotteswolds and have been 

 variously termed Bridport Sands, Yeovil Sands, Midford Sands, 

 Cotteswold Sands, etc., after the localities at which they occur. 



