354 J. BARRELL PLUVIATILE ORIGIN OP OLD RED SANDSTONE 



atecl for geological preservation, and are to be distinguished by associa- 

 tions which are commonly absent in ancient examples- 12 Thus it seems 

 clear that it is the character, the quantitative development, and asso- 

 ciations of the stratigraphic features, as well as the nature of the en- 

 tombed organic remains, which are significant rather than their mere 

 occurrence. 



In general, traditional criteria are liable to lead into error, because they 

 become accepted as axioms and are applied without further thought. 

 They lag behind the development of a science, whereas the very word 

 "research" implies the necessity of continually testing the correspondence 

 of the images of science with nature. 



Attention may now be turned to what may be regarded in the present- 

 state of knowledge as fairly definite criteria, which will be of use in 

 discussing the origin of the Old Eed Sandstone formations. 



A fresh-water origin for these deposits will be assumed as established 

 and accepted by practically all geologists, but many are accustomed to 

 thinking of the presence of fresh-water faunas and the absence of marine 

 organisms as implying nothing more than land-locked, brackish-water 

 bodies, perhaps fresh near their heads, as seen in Chesapeake Bay and the 

 Baltic Sea. The deposits, following this conception, would be called 

 estuarine. If clearly separated by a land barrier from the open ocean, 

 then the deposits have in the past been usually regarded as lacustrine and 

 taken to imply the former existence of great fresh-Avater lakes or inland 

 seas. True estuaries must have been of limited development, however, 

 in past times, since, like lakes, they are temporary features and are made 

 by a rising sealevel against a land surface previously dissected by erosion- 

 Deposits made in nearly landlocked water bodies, like the Baltic, should 

 rather be called "bay formations." Where there is some intermingling, 

 interfingering, and gradation of marine and fresh -water deposits with 

 their respective faunas, the relation which is implied is commonly that 

 of the shore of a. delta. Such a shore is marked by shifting growth, by 

 wide advances and retreats, and the inclosure of marginal lagoons of 

 various degrees of salinity. 13 



In the Old Eed Sandstone of the British Isles these marginal delta 

 phases are not so important as in the Devonian on the continent-of Europe 

 and in eastern North America. They do occur, however, in northern 

 Devon and in southern Ireland. The problem of the British Old Bed 

 Sandstone really resolves itself into the discrimination between true 



12 Joseph Barrell : Mnd-ciackp as a criterion of continental sedimentation. Jour, of 

 Geology, vol. xiv, 1906, pp. 524-568. 



13 Joseph Barrell : Criteria for the recognition of ancient delta deposits. Bull. Geol. 

 Soc, Am., vol. 23, 1912, pp. .377-446. 



