THE DEVONIAN PERIOD. 445 



culty in the way of the inclosed-basin hypothesis, in the great thick- 

 ness of the Old Red sandstone, though admitting that it is no greater 

 than that encountered in connection with other formations known 

 to have been made in inclosed seas or lakes. The thickness of the beds 

 is not very remarkable if they are interpretable as the results of 

 inclined deposition. 



The Old Red Sandstone formation, as its name implies, is mainly 

 red and largely of sandstone. It, however, includes shales as well as 

 sandstone, and the former, as well as the latter, are sometimes of a 

 color other than that indicated by the name. 



The Old Red sandstone of the British Isles is thought by Geikie 1 

 to have been deposited in several distinct basins. One of them extended 

 westward from Scotland so as to include a part of Ireland, while another, 

 that which includes the northern part of Scotland, the Orkney and 

 Shetland Islands, perhaps extended eastward to southern Norway 

 and finally still farther east to Russia. 



It is to be noted that the Catskill formation of America has some 

 features which suggest a comparison with the Old Red sandstone of 

 Britain. Both are predominantly red, both are poor in fossils, and 

 both seem to have been deposited in inclosed bodies of water. It is 

 to be noted also that the one is in the eastern part of America and 

 the other in the western part of Europe — regions where there are many 

 other striking likenesses. 



The Old Red sandstone formation of the British Isles consists of 

 two great divisions, the upper of which in many regions lies uncon- 

 formably on the lower. In such regions the lower division is conform- 

 able with the Silurian below, and the upper with the Carboniferous 

 above. Since the Silurian below is not known to be the youngest 

 Silurian, nor the Carboniferous above the oldest Carboniferous, it is 

 not certain that the Old Red sandstone is the exact equivalent of the 

 Devonian as elsewhere developed. It may include the upper part 

 of the Silurian at its base, and the lower part of the Carboniferous at 

 its top. 



The lower division of the Old Red sandstone has a maximum thick- 

 ness of something like 20,000 feet. It contains a large amount 

 of volcanic rock. In this respect it corresponds with the Devonian 



1 Trans. Roy. Soc. Edinburgh, Vol. XXVII, 1879. Also Text-book of Geology. 

 3d ed. 



