382 J. BARRELL FLUVIATILE ORIGIN OF OLD RED SANDSTONE 



westerly prolongations of this type of strata. This failure may arise either 

 from there never having been any such exceptional mortality among the fishes 

 in the western waters, or from no fortunate section having yet exposed any 

 rival to the fish-bed of eastern Fife. 37 



The well rolled quartz pebbles, the scattered fish scales, the partings of 

 clay, all testify to a dominantly fluviatile origin of the beds. Of the 

 various causes which Geikie cites as possible explanations of the concen- 

 tration of the well preserved fish fossils into certain strata, the one which 

 falls naturally into line with the interpretation given to the beds in this 

 article is that of fish crowded into pools inadequately oxygenated. 



Taken as a whole, the descriptions of various writers, although indi- 

 cating a persistence in the Upper Old Eed of fluviatile deposition, do 

 suggest a more intermittent and torrential character of rivers, implying 

 more contrasted seasonal conditions of alternate rainfall and aridity than 

 during most of Old Red Sandstone times. Time desert climates did notj 

 however, prevail, since no marked wind-facetting of pebbles has been 

 noted, the cross-bedding seems to be more fluviatile than eolian, and the 

 limy deposits are not associated with salt and gypsum. 



Geography of the British Isles in Devonian Time 



The Old Eed Sandstone formations have been described in some detail. 

 A brief consideration may now be given to the possible sources of ma- 

 terial and the indications from such lines of evidence in regard to the 

 limits of the basins as drawn in figure 1. 



In Cornwall and Devonshire is a fairly complete Devonian record, most 

 of which holds marine fossils. The basal formations are found to overlap 

 toward the south. There are indications also in Brittany of Lower "De- 

 vonian land having existed in this southwesterly direction. Toward the 

 north of Devonshire the sediments thin out against an axis of non-subsi- 

 dence located along the line of the present Bristol Channel. The open 

 Devonian shallow sea lay to the southeast and the fossiliferous rocks of 

 Cornwall and Devon represent its near-shore deposits. 



The southern part of Ireland is largely underlaid with the Glengariff 

 beds. These consist in their lower part of hard green and purple grits 

 with subordinate beds of slates, but in the higher part slates of red and 

 purple tints predominate over the grits. No fossils have been found in 

 them, though they are brought up along broad anticlinal flexures which 

 range from the west coast of Kerry to the eastern parts of Cork. Their 

 thickness has been estimated at 8,000 feet, but when due allowance comes 

 to be made for folds and faults it may prove to be much less. 38 The 



37 A. Geikie : The geology of eastern Fife. Mem. Geol. Survey of Scotland, 1902, pp. 

 57, 59. 



38 A. J. Jukes-Browne : The building of the British Isles, 1911, p. 111. 



