384 J. BARRELL FLUVIATILE ORIGIN OP OLD RED SANDSTONE 



terpretation given in figure 1 is that of several contiguous and partially 

 confluent alluvial basins. The geographic boundaries are thus taken as 

 intermediate between those given by Jukes-Browne and those by Geikie. 

 This position, it is thought, is justified on consideration of the nature of 

 the sediments, the later crust movements, the consequent denudation, 

 and, in other regions, the nature of intermontane basins. 



The Caledonian basin holds mostly Lower Old Eed deposits ; the Orca- 

 dian basin contains sediments whose age is Middle Old Eed. The two 

 basins, however, probably overlapped, deposition beginning in the north 

 before it closed in the south. An overlap of the same nature may very 

 likely have occurred between the Orcadian and Shetland basins. 



The Shetland basin has been delimited from the Orcadian, since Flett 

 and Traquair have shown that it holds a younger fish fauna. It does not 

 seem probable, furthermore, that the same individual basin should have 

 extended as far as eastern Russia, as some writers have suggested. 



This map of Great Britain in the Devonian period may be compared 

 also with the recent paleogeographical map published by Schuchert. 39 



In a larger view of Britain as a part of a Devonian continent, it is seen 

 to have lain between a sea on the southeast and a mountain system of 

 unknown extent on the northwest. It was a region furrowed with moun- 

 tains and basins. But erosion in a region of strong relief is so rapid that 

 the uplands adjacent to the basins were kept worn fairly low and the 

 basins were in general maintained in a graded condition by the rapid 

 inwash of alluvial deposits. On these basin plains shallow lakes spread 

 at times but were inconstant in character. The greater supply of sedi- 

 ment came from the northwest, the direction of the greater mountain 

 system. Only the' margins of this ancient upland remain exposed above 

 the waters of the Atlantic, showing now as the Precambrian rocks of 

 western Scotland and northwestern Ireland. 



Our more definite knowledge is restricted to the regions which were 

 originally basins and which are still above the sea. To those Devonian 

 basins, the early Tertiary basins, with their deposits in the Eocky Moun- 

 tains of the United States, may in many respects offer a close analogy. 

 In the early Tertiary the Cordillera had not yet attained its high plateau 

 character. Mountain-making was in progress and enormous quantities 

 of waste were poured as fluviatile deposits into intermontane basins. 

 Continued tectonic and igneous activity kept deforming the basins and 

 shifting the areas of deposit. Volcanic dust and breccias added impor- 

 tant quantities of material to that derived from erosion. Oscillations in 

 climate took place, the changes being on the whole toward semi-aridity, 

 but true desert conditions were of limited occurrence. The Eocky Moun- 



39 Text-book of geology, 1915, p. 716. 



