

SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 13 



The time-planes cross it as shown, the hemerae* named 

 after the various ammonites which characterise the horizons 

 being indicated in the diagram. The portions of each sand- 

 belt between a pair of dotted lines may be taken as practically 

 contemporaneous . 



The conditions thus indicate a gently rising and tilted 

 shore-line, beginning as a shallow sea in the Cotteswold district 

 whilst the area of Dorset was submerged under deeper water, 

 and becoming progressively shallow over the belt towards what 

 is now the English Channel. 



The transgression of a lithological phase across time 

 divisions marked by definite faunal assemblages may be 

 illustrated by another of the many examples from the strati- 

 graphy of Britain. Professor E. J. Garwood, in his description 

 of the Carboniferous Limestone of the N.W. of England,! has 

 discussed the evidence for the incoming of shallow-water 

 conditions during the formation of the zones of Michelinia and 

 Productus corrugato-hemisphericus in the district between Shap 

 and Ravenstonedale. Coral-bearing limestones, indicative of 

 clear-water conditions, give place to a sandy type of sedimen- 

 tation which begins at the base of Michelinia- zone at Shap, 

 and transgresses several faunal horizons in a southerly direction 

 until at Ravenstonedale it appears only at the middle of the 

 sub-zone of P. corrugato-hemisphericus , 100 feet above the base. 

 The sandy conditions also persisted to a later date (i.e., to a 

 higher horizon) in the former district. The actual thickness 

 of sandy sediment is, however, greater in the south (Raven- 

 stonedale) than in the north (Shap), despite the fact that it 

 occupied less geological time in accumulation as measured by 

 the faunal horizons. 



* The term " hemera " was introduced by S. S. Buckman (1893) : " to 

 mark the acme of development of one or more species. It is designed as a 

 chronological division . . ." (Q.J.G.S., Vol. 49, p. 481.) The term 

 " zone " is strictly a stratigraphical one. 



f Quart. Journ. Geol. Soc, Vol. 78 (1912), p. 449. 



