DEVONIAN PERIOD. 521 



that, while the Devonian strata are undoubtedly marine in 

 their origin, there seems reason to conclude that the Old Red 

 Sandstone proper was, in part at any rate, a fresh-water de- 

 posit. The two groups, therefore, might be truly contempora- 

 neous, and yet might not contain the same fossils. 



The Old Red Sandstone is pre-eminently a British forma- 

 tion, and is divisible into three groups the Lower, Middle, 

 and Upper Old Red. 



The Lower Old Red reposes with perfect conformity upon 

 the highest beds of the Upper Silurians, the two formations 

 appearing to pass into one another by an intermediate series 

 of " passage-beds," which contain large Crustaceans of the 

 family of the Eurypterids. The Lower Old Red consists 

 mainly of massive conglomerates, with sandstones, shales, and 

 concretionary limestones. Its organic remains consist chiefly 

 of plants, Crustaceans, and fishes. 



The Middle Old Red of Scotland consists of dark-grey flag- 

 stones, bituminous, flaggy shales, and conglomerates, some- 

 times accompanied by shales having irregular calcareous no- 

 dules embedded in them. The fossil remains are chiefly fishes, 

 with one Crustacean, and a few plants. 



The Upper Old Red of Scotland consists of pebbly con- 

 glomerates, sandstones, and shales, and contains many fishes, 

 a good many fragments supposed to belong to sea-weeds, and 

 some undoubted land-plants. 



In North and South Devon there occurs, underlying the 

 Carboniferous Rocks, a great series of strata which has been 

 regarded as the equivalent of the Old Red Sandstone. Though 

 certainly referable, in great part at any rate, to the period of 

 the Old Red Sandstone, it does not appear that the Devonian 

 Rocks can be regarded as the equivalent of the Old Red 

 Sandstone of Scotland. The Devonian Rocks, however, are 

 largely represented on the continent of Europe, and they are 

 richly fossiliferous ; though they do not contain any of the 

 characteristic Crustaceans, and only one or two generic re- 

 presentatives of the characteristic fishes of the Scotch Old 

 Red. 



The Devonian Rocks of Devonshire consist essentially of 

 greenish slates, alternating with sandstones, conglomerates, 

 and well-developed bands of blue crystalline limestone and 

 calcareous slates. 



In no country in the world probably is there a finer and 

 more complete exposition of the strata intervening between 

 the Silurian and Carboniferous formations than in the United 

 States. The following are the main subdivisions of the Devo- 



