lxviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [MayiCJII, 



Since the thickest part of such an accumulation is, from the 

 nature of the case, the part most liable to destruction, it must 

 follow that the thickest parts of stratified masses now preserved 

 do not afford unequivocal evidence of the exact position of sea- 

 margins. The phenomena of aggregation and erosion under the 

 conditions postulated explain for us the lenticular deposition and 

 the amount of contemporaneous erosion which characterise such 

 sediments as those parts of the Silurian rocks that were formed 

 during periods of uplift. These characteristics have been brought 

 out by the researches of Dr. Herbert Lapworth in Central Wales 

 and by Miss Elles and Mrs. Shakespear on the Welsh Border. 



In view of the fact that planes along which occur the sediments 

 actually deposited in one and the same limited period of geological 

 time — true time-planes, as contrasted with theoretical or con- 

 ventional time-planes— must cut through many kinds of sediments 

 in deposits formed under these circumstances, it is evident that 

 they will not be throughout strictly coincident with the lithological 

 divisions known as stratification. It is, therefore, necessary from 

 time to time to re-examine any of the bases of our correlation 

 in which shallow-water deposits may be concerned. Thus, in 

 tracing the variations of the Lower Oolites from the Cotswolds 

 into Lincolnshire and Yorkshire, a most useful summit-line has 

 been found and utilised in the Cornbrash, which is continuous in 

 its presence and characters from one end of the country to the 

 other. But it is improbable that this Cornbrash formation, one 

 of typical shallow-water aspect, could by any possibility have been 

 deposited contemporaneously throughout the whole of this area. 

 It has served an important stratigraphical purpose in initiating 

 correlation ; but the formation itself must almost of necessity be 

 progressively older when traced in one direction or another through- 

 out its extent. Unfortunately, J. F. Blake did not live to express 

 the conclusions to which his study of the Cornbrash fauna was 

 leading him. 



(c) 'Terrestrial Period.' 



(3) While uplift is in progress, the tectonic structures that are 

 being produced will be submitted to the denuding action of the sea, 

 which is likely to give to the whole smooth, plane-like, outlines. 

 But, so soon as there is emergence, the denuding agencies will 

 begin to adjust themselves to the growing structures, and diversity 

 in relief will be the consequence. The deposits now formed will 

 tend to accommodate themselves to this relief. For brevity, 

 they are here designated ' terrestrial deposits/ 



