APPLICATION OF TERM SILURIAN 247 



this point, Kayser, who has given much attention to the stratigraphy 

 and paleontology of the Lower Devonic, writes as follows : * 



" Paleontologically the Ludlow rocks, on the whole, are commonly very closely 

 related to the Wenlock ; most of the mollusks, many of the trilobites and other 

 fossils are common to the two formations, and only a few forms, such as Cardiola 

 interrupta and Pentamerus Knighti, are, if not entirely limited to the upper beds, at 

 least chiefly found in them. A special peculiarity of the Ludlow is the appear- 

 ance, even in the lowest beds, of numerous Eurypterids (especially Eurypterus and 

 Plerygotus— forms which become very abundant in the Old Red sandstone) — and 

 the peculiar Cephaspidse (Cephalasjns, Scaphaspis, Pieraspis, etcetera), the oldest of 

 British fishes." 



The Upper Silurian of England, as given by Geikie in 1893, is prac- 

 tically that of Murchison in 1854, with the exception of the Lower and 

 Upper Llandovery and Tarannon shales. These had been referred by 

 the former to his Lower Silurian, but by the latter are regarded as the 

 base of the Upper Silurian. The upper limits of the Upper Silurian are 

 defined by Geikie as follows : 



"Above the Upper Ludlow shales and mudstones lies a group of fine yellow, red, 

 and gray micaceous sandstones from 80 to 100 feet thick, which have long been 

 quarried at Downton castle, Herefordshire. At Ledbury these sandstones are sur- 

 mounted by a group of red, purple, and gray marls, shales, and thin sandstones, 

 having a united thickness of nearly 300 feet. Originally the whole of these flaggy 

 upper parts of the Ludlow group were called ' tilestones ' by Murchison, and, being 

 often red in color, were included by him as the base of the Old Red Sandstone, into 

 which they gradually and conformably ascend. They point to a gradual change 

 of physical conditions, which took place at the close of the Silurian period in the 

 west of England and brought in the peculiar deposits of the Old Red Sandstone. 

 There is every reason to believe that for a long time the marine sedimentation of 

 Upper Silurian type continued to prevail in some areas, while the probably lacus- 

 trine type of the Old Red Sandstone had already been established in others, and 

 that by the breaking down or submergence of the barriers between these different 

 areas, marine and lacustrine conditions alternated in the same region. The tile- 

 stones are the records of this curious transitional time. . . . 



" The evidence from fossils is equally explicit. Up to the top of the Ludlow 

 rocks, the abundant Silurian fauna continues in hardly diminished numbers. But 

 as soon as the red strata begin the organic remains rapidly die out, until at last 

 only the fish and the large eurypterid crustaceans continue to occur." f 



DOWNTONIAN 



In a very recent detailed work, " Silurian Rocks of Britain," Messrs 

 B. N. Peach and John Home % write of the uppermost Silurian rocks of 

 Great Britain as follows : 



* Text-book of Comp. Goology, 1893, p. 65. 



fjGeikie, pp. 7G0-762. 



| Memoirs Geol. Survey of United Kingdom, 1899, pp. 67-69, 80, 568. 



