AGK OF FISHES. 201 



CHAPTER XV. 



AGE OF FISHES, OB DEVONIAN AGE. 



290. Rocks. The rocks formed in this age are, in 

 North America, in the first part of the age, limestones, 

 and afterward mostly sandstones, shales, and flags. The 

 famous Xorth River flagging-stones - were made in this 

 age. So also were the Scotch flagging-stones known 

 over Europe as the Caithness flags of commerce. The 

 Devonian system of England and Scotland has the gen- 

 eral name of Old Red Sandstone, red sandstone being 

 there the principal rock of the system. The name De- 

 vonian was given to this system because its rocks were 

 early investigated by Murchison in Devonshire, England. 



291. Extent of the System in this Country. The De- 

 vonian rocks cover a large area in the interior of this 

 country, extending along south of the addition which 

 was made in the age of Mollusks to the continent as it 

 was begun in the Azoic age. Lake Erie is included in 

 this space. Lake Michigan bounds it on the west, and 

 a long, narrow arm of it extends from a little below this 

 lake far to the northwest. This area extends broadly 

 down the west side of the Appalachian coal formation 

 two thirds of its length, tapering, however, as it goes. 

 This immense Devonian field has a broad eastern branch 

 extending from Lake Erie to within a short distance of 

 the Hudson River. It is this part of the field, reaching 

 down into Pennsylvania, which is represented in the sec- 

 tion given in Fig. 101, beginning in N, the Helderberg 

 series, and ending in S, the Old Red Sandstone. Perhaps 

 some of this large extent of territory has had some of 

 the Devonian strata covered by other rocks of a later 



12 



