236 STRATIGRAPHICAL GEOLOGY 



feet), and fossils are scarce, but scales of Holoptychius occur. The 

 upper or Cornstone Group consists of sandstones and flagstones of 

 pale red or yellow colours with marls and layers of the hard 

 calcareous nodules which are known as cornstones. Fish remains 

 are abundant in places, especially at Dura Den, near Cupar, where 

 most of those mentioned on p. 226 have been obtained. This upper 

 group is just as clearly a lacustrine deposit as the lower part of 

 the series is one of accumulation in dry and wind-swept desert 

 plains. The total thickness in Fifeshire is about 1000 feet. 



Orcadian Region. The present visible extent of the Old Red 

 Sandstone has been mentioned on p. 230, but it must have a much 

 greater extension under the bed of the North Sea both in eastern 

 and northern directions, for the general dip of the beds is to the 

 north-east, and their estimated thickness is 16,000 feet. 



The whole of this thickness is ascribed to the Middle Old Red 

 Series because of the peculiar fish fauna which it contains. It is 

 divisible into four fairly distinct groups : 19 (1) the lowest consists of 

 red conglomerates, sandstones, and mudstones resting on and 

 against the Archaean rocks and largely developed near Berriedale, 

 from which place its outcrop runs N.N.W. to Sandside 

 Bay, west of Thurso ; (2) the Lybster Beds or lower flagstone 

 group dark -blue flags and shales, with occasional beds of sandstone 

 and of black, often bituminous, limestone ; (3) the Thurso Beds or 

 upper flagstones, which are of pale grey or green colours and 

 include but few limestones ; (4) the John o' Groats sandstones of 

 red and yellow tints. In tabular form the succession and thick- 

 nesses are : 



Feet. 



John o' Groats sandstones . . . about 2,000 

 Thurso Flagstone Group . . . . ,,7,000 

 Lybster Flagstone Group . . . ,, 5,000 

 Berriedale Beds ,,2,000 



About 16,000 



Fish remains are most abundant in the Flagstone Beds, the 

 lower group yielding Thursius macrolepidotus, Dipterus macropterus, 

 and Osteolepis ; the higher beds contain abundant remains of Ooc- 

 costeus (C. decipiens and 0. minor) with many others (see p. 226). 

 The highest group has yielded plant remains, such as Lepidodendron 

 gaspeanum, Palceopteris Browni, and Calamites transitionis, which 

 occur also in the Devonian of Canada. 



A fish bed of special importance occurs at Achanarras, some 

 10,000 feet above the base of the formation, while a similar 

 assemblage of species has been found in the Moray area not very 

 far above the base, and if the two beds are really on the same 



