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plants, ostracods, phyllocarids, eurypterids, and fishes. He says : " There is to 

 be noted the absence of ccelenterates, brachiopods, echinoderms, and trilobites, 

 representatives of which are found in the true-marine Ludlow rocks. This absence 

 is as striking as the lingering presence of a few marine types. On the other 

 hand the ostracods, eurypterids, and fishes are groups which are found also as 

 characteric fossils in clearly freshwater deposits as well as in brackish water or 

 marine deposits." The same remarks apply as pertinently to the fauna of 

 the Old Red Sandstone itself. The Downtonian probably represents deltaic 

 deposition on a rising shore-line. 



The Old Red Sandstone conglomerates have been ascribed to a littoral origin 

 or to the beaches of lakes. But this view cannot now be held in view of the 

 demonstration by Barrell and others of the insignificance of littoral deposition 

 both in thickness and areal extent, and of the numerous chances against 

 the preservation of littoral sediments {i.e. deposited within tidemarks) in the 

 geological record. Even if the term " littoral '' be extended to cover the deposits of 

 shallow seas, this mode of deposition is inadmissible in the case of the Old Red 

 Sandstone in view of the great thickness of its conglomerates and its persistent 

 records of subaerial exposure. Even if a great lagoon or marine playa like the 

 Rann of Cutch (8,000 square miles) be postulated, the difficulty of the main- 

 tenance of the nice balance of geological forces necessary to ensure conditions of 

 subaerial exposure for long periods of time and during thick sedimentation must 

 negative the idea of marine origin. 



For a close analogy to Old Red Sandstone conditions of deposition in a much 

 later geological period, Prof. Barrell cites the Tertiary deposits laid down in 

 the intermontane basins between the growing ranges of the North American 

 Cordillera. At the present time regions which have similar climatic and 

 depositional conditions to those of the Old Red Sandstone are widespread. In 

 particular South America east of the Andes and south of the Amazon basin, 

 Central Africa except the Congo basin, and the southern alluvial basins and 

 deltas of Indo-China, may be cited. 



The study of sedimentation in relation to past climates and topographies has 

 led Prof. Barrell to another fruitful line of thought, which concerns the effect of 

 Silurian-Devonian climates upon the evolution of air-breathing vertebrates (see 

 Barrell, op. cit.). He observes truly that the study of the rise of any group of 

 animals involves not only biological considerations, but also an analysis and 

 evaluation of the environment to the fluctuation of which they made the response 

 that led to evolution. There is therefore a side of the story of life which belongs 

 to physical geography, the study of past climates, geographies, and conditions 

 of deposition ; and it is from this point of view that the influence of the Middle 

 Palaeozoic climates upon the development of amphibia has been studied. 



Briefly the rather complex argument is as follows : it is probable that fishes 

 arose in land waters where they constituted primarily a river fauna. If they arose 

 in the sea it is difficult to understand the comparative absence of their fossils from 

 truly marine sediments, and their comparative abundance in the deposits of 

 brackish and protected waters, and in sediments of truly continental origin. In 

 the early Palaeozoic the record of land deposition is very meagre, and con- 

 sequently it is only in the opening phases of the Devonian that fish fossils are 

 found in great abundance. At this time the acanthodian sharks were the 

 dominant fishes in the continental waters ; but in the later Devonian the sharks 

 had disappeared from the fresh waters and the crossopterygians reigned in their 

 stead. Their ability to use air adapted them better for life in a climate marked 



