CONCLUSIONS ON DEVONIAN CLIMATE IN BRITISH ISLES 385 



tains, then, from the tectonic standpoint, may offer an instructive anal- 

 ogy, whereby the Old Keel Sandstone deposits of Britain may be inter- 

 preted in the light of the knowledge gained of the more recent past. 



General Conclusions on Devonian Climate in British Isles 



The absence of even discontinuous and local beds of coal implies a 

 rather complete desiccation of the marshes during the dry season. The 

 same conclusion is reached by the prevalence of mud-cracked surfaces in 

 the finer-grained deposits, shown especially in the Orcadie basin. Many 

 of the conglomerates are rather highly feldspathic, and both the sand- 

 stone and flagstone groups are characterized by the presence of a large 

 amount of limestone deposited from fresh waters. This assemblage of 

 characteristics marks especially the floodplain deposits of regions with a 

 semi-arid climate or at least with a pronounced dry season, such as the 

 upper floodplain of the Ganges. 



The significance of the dominantly red color has been discussed. It 

 implies the regular recurrence of seasons of dryness, effective for main- 

 taining the iron in fully oxidized form. 



Furthermore, it was noted that the flagstone groups, in places char- 

 acterized by their mud-cracks, were also rich with fish remains, and that 

 on certain surfaces the fossils are crowded thickly together and preserved 

 in such detail as to show that they died where their remains are found. 

 The natural explanation in connection with the hypothesis of floodplain 

 origin is found in the existence of seasons of drought which first concen- 

 trate the fish into shallow lakes on the floodplain and, by further evapora- 

 tion of the water bodies, kill them. For death to take place complete 

 desiccation is not necessary, but merely sufficient concentration to make 

 the waters too crowded and foul for respiration. Such a sheet of water, 

 deficient in oxygen and protecting the remains from complete decomposi- 

 tion, may explain the presence of much of the bitumen found associated 

 with the fossils and into which they are sometimes bodily transformed. 



These features indicating that the climate had neither the humidity 

 nor the seasonal uniformity which marked the coal-making portions of the 

 Carboniferous limit the climate in one direction. Facts which tend to 

 delimit it in the other direction are complete absence of salt and gypsum 

 deposits; gray color and bituminous composition of the flagstone groups; 

 presence of ferns, logs of trees, and roots; presence in some of the basins 

 of thin coaly lamina? and surfaces of sedgy, matted vegetation. 



These facts converge from the two directions of dryness and wetness 

 and serve to narrow down the determination of the climatic conditions in 

 the regions of accumulation, In some respects they may appear at first 



