CRITERIA AS TO MODES OP ORIGIN OF SEDIMENTS 357 



with the original stratum; and weathering, furthermore, destroys such 

 strata very rapidly unless the shales are interlaminated with sandstones. 

 Rain-prints must be separated from the pits made by escaping marsh gas 

 and are more usually marked by a spattered surface of the mud than by 

 a few concave depressions. Root-marks should show a branching pattern 

 and finer tendrils given off from the larger marks. Footprints may be 

 very obscure, but the test in that case is the regularity of recurrence on 

 the stratum, owing to the stride of the animal and its regularity. 



Finally, a criterion of special application to much of the Old Red Sand- 

 stone deposits is found in the nature of the conglomerates. 



Conglomerates now forming which are both thick and wide-spread, as 

 Blanford and Bonney have shown, are observed to be of fluviatile and 

 not littoral origin. This is because rivers are able to carry gravels far 

 out over a subsiding river plain; but the waves, on the contrary, tend to 

 keep gravel banked in the zone of the shore. During an advance or re- 

 treat of the sea, basal conglomerates may be widely spread, but they are 

 thin and often wanting, reaching their greatest development among 

 islands or along an irregular rocky shore able to withstand the waves for 

 some time during a rising sea. A maximum limit to wide-spread basal 

 marine conglomerates seems to be 100 feet, and therefore broad con- 

 glomerate formations of greater thickness are evidences of terrestrial 

 accumulation. 14 - They may, of course, be of terrestrial origin also when 

 thinner and more limited; but in that case, so far as these criteria deter- 

 mine, they may also be marine or lacustrine. 



Other characters which are, under certain circumstances, of importance 

 are found in the detailed structures and associations of the conglomerate 

 beds. Gravels which have been carried considerable distances by power- 

 ful rivers may be as well sorted and as thoroughly worn as are the wave- 

 worn gravels of the shore, but weak rivers carry their debris especially 

 during great floods. Coarse and fine are swept along together; sorting 

 and wear are less perfect ; streaks of gravel are swept out from the basin 

 margins and intercalated with dominant sandstones and even with beds 

 of shale. River gravels are shingled by the currents so that the longer 

 diameters of the pebbles dip upstream, giving a faint appearance of false 

 bedding, which on the average, unlike the false bedding of sandstone 

 strata, dips toward the basin margin. Shore gravels, on the other hand, 

 are developed parallel to the shore. The onshore waves have a greater 

 force than the undertow and the shingling dips away from the shore, or 

 runs out laterally from protruding headlands. 



14 Joseph Barrell : Some distinctions between marine and terrestrial conglomerates. 

 Abstract. Bull. Geol. Soc. Am., vol. 20, 1908, p. 620. 



