376 J. BARRELL FLUVIATILE ORIGIN OF OLD RED SANDSTONE 



necessary for the continuance of deposition of subaerial conglomerates is 

 the maintenance of rapid erosion to supply the material and river volume 

 and grade sufficient for its transportation. The conditions for the con- 

 tinuance of deposition of subaqueous conglomerates, on the contrary, 

 depends on the permanent existence of a shore of suitable materials and 

 a depth of water just such that waves and currents may transport gravel. 

 The great Pleistocene gravel deposits of the continental interiors testify 

 to the magnitude of the first class of conglomerates, while the limited 

 distance to which present gravels are transported from shores, as shown 

 by hydrographic charts, indicates the relative insignificance of the second. 



The pebbly sandstones far from the base of the Caithness section and 

 at distances of many miles from the regions of erosion are highly sug- 

 gestive of subaerial conditions, while conglomerates, such as those of the 

 Moray Firth, forming thick masses, coming in again and again, and 

 overlying formations of clay and shale, may be taken as fairly conclusive 

 evidence of terrestrial deposition. 



Significance of colors. — Variegated colors, especially where these alter 

 from one stratum to another and even within a single stratum, are 

 highly characteristic of continental deposits, as Walther has pointed out. 

 Bed sediments have been deposited to a limited extent under permanent 

 water bodies, either salt or fresh, but are characteristically of terrestrial 

 origin. Even where deposited under a water body the color is due to 

 previous thorough oxidation of the iron while exposed to the air without 

 the usual opportunity for later subaqueous deoxidation. 



The climatic conditions which may give rise to red tones in consolidated 

 deposits are rather broad. As argued elsewhere, 30 the presence of red 

 merely implies the existence at the time of deposition of either temperate 

 or torrid climate with a dry season sufficiently marked to permit periodic 

 aeration of the fLoodplain soil. 



The dominantly red color and variegated character of the conglomerates 

 and sandstones is therefore strongly suggestive, though not conclusive in 

 itself, of terrestrial origin under rather broadly limited climatic con- 

 ditions. 



The presence of the dominantly gray and blue colors in the flagstone 

 groups with their high content of bitumen implies that, if of terrestrial 

 origin, they were deposited under conditions which did not permit suffi- 

 cient aeration and oxidation to eliminate the organic matter and oxidize 

 the iron. Such conditions are broadly developed over the lands in the 

 lowest portions of river floodplains, more or less continuously in a swamp 

 condition, more especially in climates without marked dry seasons, or in 



30 Joseph Barrell : Jour. Geol., vol. xvi, 1908, pp. 285-294. 



