GEOGRAPHY OF BRITISH ISLES IN DEVONIAN TIME 383 



Upper Old Eed is also present and graduates upward into the Coomhola 

 beds and the Carboniferous slate. The Coomhola beds are alternations 

 of gray and brown grits with bands of dark gray slate and hold a marine 

 fauna. The unfossiliferous state and physical nature of the Glengariff 

 suggests strongly that the beds are not of marine origin. It appears 

 likely that they were mostly laid down on delta plains which toward the 

 southeast were built out into the interior sea. In the Lower Devonian 

 the South Irish basin of deposit was restricted, but in the Upper De- 

 vonian sedimentation advanced widely over central Ireland. Figure 1 in- 

 dicates the relations of the basins of Lower and Middle Old Red Sandstone 

 and the boundary of the South Irish basin as given by Jukes-Browne. 



The Lower Old Red sediments which made the Glengariff beds did not, 

 however, come in large measure from central Ireland, since the post- 

 Carboniferous erosion of anticlines reveals the fact that the Upper Old 

 Red sediments rest on Silurian strata. In southeastern Ireland, south of 

 Dublin, there is a large granite massif intruded probably during the 

 Lower Devonian into Ordovician rocks. This resulted doubtless in some 

 subsequent erosion from this region, but for adequate sources of- supply 

 for the great volume of the Glengariff beds, especially for the parts of 

 earlier date, it would appear that we must look to Precambrian areas to 

 the south, west, and northwest now mostly concealed by ocean waters. 



The Old Red of the Welsh basin rests on Silurian rocks. Silurian and 

 Ordovician strata mantle, furthermore, the greater part of Wales. If the 

 Old Red had been derived from these formations, either folded or un- 

 folded, it would seem that this Devonian erosion should have destroyed 

 much of the formations in areas where they are now exposed. Therefore 

 probably the greater part of the Old Red sediment could not have come 

 from this region of earlier Paleozoic rocks. The region of northwestern 

 Wales and Saint Georges Channel must be looked to as a chief source of 

 supply, though eastern England may also have contributed; the evidence 

 from that region is, however, buried beneath younger strata. 



In the Caledonian basin are great quantities of igneous outpourings, 

 so that the amount of surrounding erosion which is implied by the sedi- 

 ments is much less than the volume of deposit. Later denudation has 

 evidently, however, removed the Old Red from great areas, reducing the 

 apparent volume of the original deposits, since the boundaries are now 

 largely faults against which the beds dip with great thicknesses. Isolated 

 remnants on the Firth of Lome and in northernmost Ireland indicate 

 important extensions of the original basin, or the existence of closely ad- 

 jacent basins, in these regions. The character of much of the sediment 

 does not suggest a very distant source. Jukes-Browne has shown one 

 great lake, whereas A. Geikie believed in several smaller lakes. The in- 

 XXVIII — Bull. Geol. Soc. Am., Vol. 27, 1915 



