DESCRIPTION OF THE FORMATIONS 365 



greywackes, and brown carbonaceous shales yielding fossils resemble litho- 

 logically Upper Silurian rocks, still the dominant feature of the series as a 

 whole is red and yellow false-bedded sandstones. It seems just to infer that 

 the Downtonian fish-band and the associated mudstones and greywackes are 

 marine deposits, for some of the eurypterids found in the latter strata are the 

 associates of graptolites in the Wenlock Rocks of the Pentland Hills, and of 

 brachiopods (Lingula minima) in the Ludlow Rocks of Lanarkshire. More- 

 over, the occurrence of the Polyzoon, Glauconome, together with Spirorbis and 

 sponges, likewise points to the marine origin of some of the Downtonian 

 strata. The red and yellow false-bedded sandstones, on the other hand, evi- 

 dently herald those conditions which prevailed during Old Red Sandstone 

 time, when the open sea gave place to brackish water or inland lakes." 20 



Peach and Home call this fauna marine, but there is to be noted the 

 absence of ccelenterates, brachiopods, echinoderms, and trilobites, repre- 

 sentatives of which are found in the true marine Ludlow rocks. This 

 absence is as striking as the lingering presence of a few marine types. 

 On the other hand, the ostracods, eurypterids, and fishes are groups which 

 are found also as characteristic fossils in clearly fresh-water deposits as 

 well as in brackish water or marine deposits. The conditions of deposi- 

 tion of the fossiliferous zone would seem therefore to represent tbe re- 

 curring but temporary existence of brackish water, permitting for a time 

 tbe incursion of a fauna related more to the open sea than to the land. 

 It seems to have been but a temporary condition, since the other forma- 

 tions, as noted by Peach and Home, are suggestive of the true Old Eed 

 Sandstone conditions. The descriptions suggest the current-laid sedi- 

 ments of streams rather than a condition of lakes or bays. 



Two hundred miles south, in Staffordshire, limited outcrops of upper- 

 most Silurian rocks have been recently discovered which show brachiopod 

 faunas. 21 One hundred miles northeast of Lanarkshire the Downtonian 

 rocks outcrop again along the shore at Stonehaven beneath the true Lower 

 Old Eed. Hickling gives these basal Stonehaven beds a thickness of 1,500 

 feet and notes the occurrence at the base of breccia, followed by fine red 

 sandstone, with numerous thin bands of bright red shale. Higher up 

 several beds of the red marly shale, 50 to 100 feet in thickness, are inter- 

 calated with light red or yellow sandstones and fine grits with some bands 

 of gray sandstone and grit. The series of beds is distinctly fine in char- 

 acter as compared with the overlying mass of the Old Red. 22 The base 

 at this place is regarded by B., Campbell as an unconformity and not a 



20 Peach and Home : The Silurian rocks of Britain. Mem. of the Geol. Survey of the 

 United Kingdom, vol. i, Scotland, 1899, pp. 67-69. 



21 W. W. King and W. J. Lewis : The uppermost Silurian and Old Red Sandstone of 

 South Staffordshire. Geol. Mag., Decade 5. vol. ix, 1912. pp. 437-443. 



22 The Old Red Sandstone of Forfarshire, Upper and Lower. Geol. Mag., Decade 5, 

 vol. v, 1908, p. 399. 



