CRITERIA AS TO MODES OF ORIGIN OF SEDIMENTS 353 



tion as on the soundness of the principles of interpretation- Especially 

 important is the quantitative evaluation of the significance of various 

 characteristics, since it is true that some lacustrine conditions will accom- 

 pany fluviatile deposition and vice versa. Loose sand may be subjected 

 to wind action and build dune structures even in humid climates. Semi- 

 aridity is a condition which will permit the development of features which 

 are also found in the most arid climates, and yet semi-aridity does not 

 exclude the existence of abundant life. Semi-aridity is more widely asso- 

 ciated with seasonal rainfall than with deficient precipitation throughout 

 the year. The rainfall may be as abundant during the rainy season as 

 that of truly humid climates. Fluviatile deposition is vitally distinct 

 from either lacustrine or desert conditions, and semi-aridity is a type of 

 climate equally distinct from either normally humid climates, on the one 

 hand, or arid climates on the other. Yet it is fluviatile deposition under 

 a semi-arid climate which is here held to have characterized the laying 

 down of the Old Red Sandstone. This interpretation is essentially new 

 and lies between that of the lakes of Austen and Geikie and the deserts of 

 Walther and Goodchild. 



As stated to a considerable degree in a previous paper, 10 many features 

 from which an observer may gain an impression as to the mode of origin 

 of a deposit are really not definite proofs. Thus ripple-marked, cross- 

 bedded, and fossil-barren deposits may be developed either beneath a 

 permanent water cover or on river plains, though doubtless the quantity 

 and quality of their development differ in the two cases. 



Red beds have been regarded by some, because of their color, as evi- 

 dences of terrestrial deposition ; by others, of seas barren of life. Again, 

 they have been cited as indications of a deeply decayed regolith, of a 

 humid climate, or of aridity. The present writer holds that redness in 

 rocks may in fact accompany any of these conditions, and is not therefore 

 a criterion by itself of any one. 11 



Mud-cracks and conglomerates have been cited usually as evidences of 

 ancient tidal flats and beaches, but are now observed to occur chiefly in 

 river deposits of continental interiors. Those formed in the littoral zone, 

 furthermore, except on the fronts of deltas, are rather unfavorably situ- 



the mode of origin of formations in the British Isles and the special feature of the paper 

 is the application of these criteria, it seems desirable to give a fuller review of the 

 principles on which they rest. This is because it is hoped that the subject will he of 

 interest to British geologists, to many of whom the papers on which this discussion 

 rests may not be readily accessible. 



10 Joseph Barrell : The Upper Devonian delta of the Appalachian geosyncline. Amer- 

 Jour. Sci., vol. xxxvi, 1913, pp. 436-440. 



11 Relations between climate and terrestrial deposits. (The climatic significance of 

 color.) Jour. Geol., vol. xvi, 1908, pp. 285-294. 



