TRENTON PERIOD. 207 



thickness in Pennsylvania, — probably 2000 feet or more. In eastern Tennessee 

 the thickness is more than double that in the central basin of the State, being 

 at least 500 or 600 feet (Safford). 



(c.) Arctic region. — The Trenton limestone has been identified in the Arctic on 

 the west shore of King William's Island, at Fury Point on North Somerset, on 

 the east and west sides of Boothia. The Boothia rock is a dolomite, containing 

 carbonate of lime 54.92, carbonate of magnesia 42.57, clay and oxyd of iron 

 2.51. 



3. Minerals. — The lead-mines of Wisconsin and the adjoining region are 

 situated in the Galena limestone, — a rock named from the mineralogical designa- 

 tion of the common ore of lead, galena. The ore occurs in large irregular beds 

 or extended masses, sometimes spreading like veins, though not properly of this 

 nature. The lead-region of Wisconsin and Illinois, according to Owen, is 87 

 miles from east to west and 54 from north to south ; and throughout much of 

 this region traces of lead may be found. The beds resemble in position the 

 lead-mines of Missouri; but the latter occur in a limestone of the Calciferous 

 epoch. These mines of the Upper Mississippi have been the subject of a recent 

 report by J. D. Whitney. 



2. European. 



Rocks of the Trenton period occur in Great Britain and many- 

 parts of Europe ; and by their general distribution they show that 

 they have the same continental character as in North America. 



In England, the rocks, instead of being limestones, are almost solely shales 

 and shaly sandstone (flags), with only thin beds of limestone. They include 

 the Llandeilo flags and shale, which are many thousand feet thick, succeeding 

 to which, as the following part in the series, are the Caradoc sandstone of 

 Shropshire and the Caradoc or Bala formation of Wales. The latter are sup- 

 posed to represent the American Hudson period. Among the Bala beds are 

 some thin layers of limestone. The thickness of the Lower Silurian of Great 

 Britain above the base of the Llandeilo formation of Wales is estimated by 

 Murchison at 18,000 feet. 



In Spain, also, there are schists and sandstones, with some limestones. In 

 Scandinavia there are limestones overlaid by slates and flags ; and in Russia 

 and the Baltic provinces, mainly limestones. 



It thus appears that along the border regions of the European continent, as 

 in England and Spain and Scandinavia in part, the rocks are mainly sandy or 

 argillaceous, while over the interior, limestones abound. 



I. Life. 

 1. American. 

 1. Plants. 

 Sea-weeds are the only fossil plants. Two of the species -are 

 represented in figs. 262, 263. 



