lxxii PROCEEDINGS OF THE GEOLOGICAL SOCIETY . [May I9II, 



coastal deposits furnish us with the most precise data that we possess 

 concerning the physical changes of the period of subsidence, and it 

 is very unfortunate that it is just this class of deposits which is 

 most susceptible to destruction and removal later in its history. 



When, after partial or complete submergence, old high lands 

 have again become resistant high land, there is a great tendency 

 for streams to grow between them and their enwrapping sediments. 

 It is here that the marginal and coastal types of sediment will 

 occur, and hence there will be considerable risk of their ultimate 

 destruction and disappearance. The Severn, the Dee, the Ouse t 

 and the Trent have robbed us of many valuable data which might 

 have given us better knowledge of the local geography throughout 

 the whole of Mesozoic times. But fortunate chances in the Mendips 

 and South Wales, in Devon and the Eden Valley, in parts of Shrop- 

 shire and in Charnwood Forest, have left precious relics of marginal 

 deposits of many ages, from Llandovery up to Cretaceous times. 

 At these isolated spots the facts enable us to extend in ima- 

 gination the margins of Mesozoic and earlier seas against the 

 older land - masses, and allow us to assure ourselves that the 

 science of palceogeography is not so hopeless as some would have us 

 believe. We have the old pebble-beaches, the screes, the cavern 

 and fissure-deposits, the washed-off vegetation and land-dwellers, 

 and, in certain cases, of which those already discovered are perhaps 

 only a foretaste, considerable areas of the enveloped landscapes. 



In the case of old high lands, which after complete burial have 

 failed to re-emerge from their enveloping sediments, more con- 

 tinuous evidence is in existence ; though, from the nature of the 

 case, it is more rarely recoverable. Our knowledge in such cases 

 is limited to facts collected from borings and to inferences drawn 

 from the conditions of exposed deposits earlier or later in date. 

 Of the former our growing knowledge of the Armorican massif is 

 an excellent type ; while of the latter Prof. Kendall's utilisation 

 of the variation and the relations of the Jurassic and Cretaceous 

 sediments of the Eastern Midlands to infer the extension and 

 continued instability of the Charnian axis is a brilliant example. 



(e) 'Thalassic Period/ 



(5) The deposits of a ' thalassic ' period succeeding a period of 

 uplift demand only passing notice. The successive encroachments 

 of the sea bring an increasing proportion of fine-grained and 

 organic deposits, with partial and (it may be) complete envelopment 

 of the old land-areas. 



