DEVONIAN SYSTEM. 57 



in the Gaspe series, and in cavities of an amygdaloidal greenstone at Tar Point, 

 which has hardened in some instances to the consistency of pitch, and from its 

 peculiar odor the name Tar Point was given to the locality. The source of this oil 

 is from the fossiliferous rocks or shales beneath, and exudes from an anticlinal. No 

 good well has, however, been discovered by boring in these rocks. 



§ 116. With this Group the Upper Silurian closes, because we have another 

 stratigraphical and palseontological chasm, and have arrived at the top of the 

 System as established by Murchison. The absolute want of conformability, with 

 the overlying rocks, is everywhere apparent, and an age of time is therefore un- 

 represented in the geological column. 



CHAPTER XVIII. 



DEVONIAN SYSTEM. 



§ 117. The Devonian was named in 1837, by Murchison, from Devonshire, in 

 England. It has greater thickness, and is capable of more subdivisions based upon 

 its fossils in this country than in any other part of the world. It is subdivided in 

 ascending order as follows : Oriskany Group, Upper Helderberg Group, Hamilton 

 Group, Portage Group, Chemung Group, and Catskill Group. 



§ 118. It commences with a sandstone formation, after which it consists 

 principally of limestone and shales. It is unconformable with the Upper Silurian 

 at all places, except possibly Gaspe, Canada, where the sediment seems to have 

 been regularly deposited from one age to the other. Its greatest development is 

 in New York and Pennsylvania, where mechanical detritus accompanies the ma- 

 rine deposits. During this era land-plants became abundant, and fish swarmed 

 within the seas, while the Archipelago, which had existed in the Silurian era, be- 

 gan to assume somewhat the outlines of a continent, though by no means such as 

 we now behold. Corals, Crinoids, Brachiopods, Gasteropods, Cephalopods, Lamel- 

 libranchs, and Crustaceans were abundant, while Cystideans became extinct. It 

 was a long and glorious era, marked by more progress in animal and vegetable 

 organisms than characterized earlier ages. The plants increased in number of 

 genera and species from the Lower to the Upper Devonian, until the flora pre- 

 sented a strong resemblance to that of the Subcarboniferous, especially in the prev- 

 alence of Gymnosperms and Cryptogams, though very few species are identical in 

 the two Systems. It is everywhere unconformable with the Subcarboniferous. 

 The masses and dykes of intrusive granite in Nova Scotia, which penetrate all the 

 rocks older than the Subcarboniferous, belong to the close of the Devonian. The 

 carbonaceous shales of this System exceed in thickness those of any other System 

 of rocks, and, as a result, they are the chief oil and gas producing rocks on the 

 continent. Very valuable iron ores and manganese ores occur in this System in 

 different States. In Virginia huge masses of manganese are found imbedded in 

 exposed sandstone ledges, where the supply seems to be practically inexhaustible. 



