364 J. BARRELL FLTJVIATILE ORIGIN OF OLD RED SANDSTONE 



the Old Bed, we may turn attention to those detailed stratigraphic char- 

 acters -which go to show the conditions of origin. 



UPPERMOST SILURIAN — DOWNTONIAN FORMATIONS 



"Rocks. — The series of strata grouped under the term Downtonian has 

 hitherto been regarded as of Lower Old Red Sandstone age, owing to the 

 prevalence of red and yellow sandstones and shales which are the prominent 

 feature of that formation. The recent discovery by the Geological Survey, in 

 shales and mudstones intercalated in these sandstones, of a marine fauna 

 which in some respects is identical with that of the underlying Ludlow Rocks 

 has led to a revision of the classification hitherto adopted. These passage 

 beds are now viewed as forming the highest subdivision of the Upper Silurian 

 rocks. They may be briefly tabulated in descending order as follows : 



"Lower Old Red Sandstone, the basal bed being a coarse conglomerate or 

 conglomeratic sandstone, with pebbles composed mainly of greywacke derived 

 from the Southern Uplands. 



"Unconformability in the Pentland Hills and in Ayrshire, apparent con- 

 formability in Lanarkshire. 



C 4. Chocolate-colored sandstone. 



Conglomerate with quartzite pebbles derived from the High- 

 lands. 



Green and red mudstones with bands of greywacke and brown 

 flaggy carbonaceous shales with fishes and eurypterids. 



Red and yellow sandstones and mudstones, underlain in the 

 Hagshaw Hills by a fine conglomerate of local occurrence, 

 resting conformably on Upper Ludlow Rocks. 



"Fossils. — The organic remains, which are restricted to Zone 2 of the fore- 

 going series of strata, consist of plants, ostracods, phyllocarid crustaceans, 

 eurypterids, and fishes. 



"Among the fragments of plants obtained from this horizon, Mr. Kidston 

 has identified Pachytheca and one specimen as belonging to the genus Parka, 

 though of a different species from P. dccipiens. The ostracods are represented 

 by Beyrichia, a form which is common in the Upper Silurian rocks of the 

 Southern Uplands ; the phyllocarid crustaceans by Ceratiocaris. Most of the 

 genera of eurypterids found in the Wenlock and Ludlow Rocks in Lanarkshire 

 and the Pentland Hills, viz. : Eurypterus, Pterygotus, Slimonia, Stylonurus, 

 have been obtained from the Downtonian fish-band (Zone 2 of above table). 



"The most striking pala?ontological feature, however, is the remarkable as- 

 semblage of fishes procured from this horizon which are wonderfully complete 

 when carefully extracted from the carbonaceous shales. Dr. Traquair has 

 identified in the collection of the Geological Survey five genera of fishes, four 

 of which are new, and seven new species. One genus (Thelodus) is common 

 to the Upper Ludlow Rocks of Lanarkshire and Wales and to the Lower Old 

 Red Sandstone of Forfarshire and Oban. 



"Conditions of deposition. — The Downtonian strata indicate a marked change 

 in the phases of sedimentation from those which obtained in Ludlow and Wen- 

 lock time in the south of Scotland. While it is true that the green mudstones, 



Downtonian.. 



