DESCRIPTION OP THE FORMATIONS 363 



area appear to represent a distinct flora, but the vertebrate fauna shows 

 relationships to the Upper Old Eed; consequently it may be the lowest, 

 or one of the lowest, zones of the Upper Old Eed. The well determined 

 genera are A sterol epis and Holonema. 19 



The typical Upper Old Eed is found resting with angular unconformity 

 on the Middle Old Bed and all older rocks, but passes conformably into 

 the Carboniferous rocks above. The basins of deposition only partly 

 corresponded with those of the older red sandstones. The rocks are 

 characteristically red and yellow sandstones and conglomerates. Bothrio- 

 lepis major and HoloptycMus nobilUsimus are typical species. 



From this series of unconformities and shiftings in regions of deposi- 

 tion it is clear that great crust movements were intermittently in progress 

 in this zone during latest Silurian and all of Devonian time. These were 

 accompanied by much igneous activity, since lava flows, breccias, and 

 volcanic ash make up large portions of the sediments, especially of the 

 I ower Old Eed. Uplifts along certain axes must have been in progress 

 to have supplied the great quantities of coarse waste. Downsinking of 

 adjacent areas must have accompanied the former in order to permit such 

 thick accumulations. Fault zones and step-faulting are implied ; perhaps 

 also true folding. In these ways may be partly explained the remarkable 

 variations in thickness of formations, the disappearance of others, such 

 as the British Survey has recently shown to exist in southern Wales and 

 in northern Devon and Somerset. Here an east-west axis runs through 

 the Bristol Channel. On the northern margin of this axis in Glamorgan 

 the total thickness of the Old Eed sandstone is reduced to 400 feet, but 

 on the northern outcrop of this southern Welsh area the thickness swells 

 to 3,000 or 4,000 feet. Similar thicknesses are found in northern Devon, 

 on the south side of this axis. It is not easy to assume a narrow and 

 lofty mountain range between these two basins, enduring without rejuve- 

 nescence and gradually buried by the mantle of sediments, for the erosion 

 which would provide this thickness of sediments would also destroy the 

 axial character and height of the mountains. Differential, intermittent 

 uplift of this axis and subsidence of the adjacent belts during sedimenta- 

 tion seems a probable relationship. In the structural complex of ridges 

 and basins we maj recognize considerable resemblance between the British 

 Isles in Devonian time and the Cordilleran area of the United States in 

 the Tertiary. The analogy is instructive not only from the structural 

 standpoint, but from the physical conditions of sedimentation as well. 



With this introduction on the general character and relationships of 



18 John R. Flett : On the age of the Old Red Sandstone of Shetland. Trans. Roy. Soc. 

 Edinburgh, vol. xlvi, pt. ii, 1908, pp. 313-320. 



