362 J. BARRELL FLUVIATILE ORIGIN OP OLD RED SANDSTONE 



These are the kinds of tests which must be used in the study of the 

 mode of origin of the formations of the Old Ked Sandstone. 



Description of the Old Bed Sandstone Formations 

 general relations 



In order that each reader may possess the basis for an independent 

 conclusion on the conditions of origin of a series of formations, detailed 

 descriptions must be given of the significant characters. Since the con- 

 ditions may shift from stage to stage, these descriptions must cover the 

 whole series with some degree of completeness. 17 



The results of continued investigations by British geologists have shown 

 that the Old Bed Sandstone conditions of deposition began in Scotland 

 and in north England in uppermost Silurian times, the marine rocks of 

 the Ludlow passing conformably, by oscillation of conditions, into barren, 

 red and yellow, cross-bedded sandstones, with some conglomerates, and 

 red to gray or green mud-stones. Goodchild states that a violent uncon- 

 formity separates the base of the true Old Bed of Devonian age from 

 these basal rocks, which the British Survey has named Downtonian, but 

 for which Goodchild prefers the name Lanarkian, from their typical ex- 

 posure in Lanarkshire, in southern Scotland. 18 



Above the Downtonian and unconformable to it comes the Lower Old 

 Bed Sandstone, characterized by red and purple sandstones, gray sandy 

 flagstones, and coarse conglomerates. The most characteristic fossils are 

 Cephalasp-is, Pteraspis, Climatius, and Pterygotus. It is typically shown 

 south of latitude 57°, especially in the Caledonian area, according to 

 Geikie, some 20,000 feet of beds being there exposed. 



Lower Old Bed deposition was closed by disturbances which shifted the 

 regions of great subsidence to the north, especially around the Moray 

 Firth and in Caithness. These are the Orcadian formations, estimated 

 to reach as much as 17,000 feet in thickness. The typical fossils are 

 Estheria, Dipterus, Osteolepis, Homosteus, Mesacanthus, Coccosteus, 

 Ptericlithys, etcetera. 



The Middle Old Bed is in turn separated by a profound unconformity 

 from the rocks stratigraphically above, which constitute the Upper Old 

 Bed, but recently Flett has shown that the Old Bed of the Shetland 

 Islands bridges to some extent this hiatus. The plants of the Shetland 



17 Descriptions of the features significant of origin will be quoted more fully for the 

 benefit of American readers than if this article were to be printed in a British journal, 

 for the reason that much of the original literature may be difficult of access to many 

 American geologists. 



18 J. G. Goodchild: The older Deutozoic rocks of Great Britain. Geol. Mag., Decade 5, 

 vol. i, 1904, pp. 591-602. 



