624 HISTOEICAL GEOLOGY. 



and ornamental marble in West Rutland, Dorset, Pittsford, etc., Vt. ; archi- 

 tectural marble in Lee, Mass., Canaan, Conn., Westchester County, N.Y., and 

 in Pennsylvania ; the Trenton, a beautiful mottled brown and reddish brown 

 marble in east Tennessee in Hawkins County and Knox County ; the lighter 

 spots in it are delicate Corals {MonticuUpora, Stenopora, etc.). 



Iron ore. — The valuable iron ore, limonite, occurs in great beds along the junction 

 of the Lower Silurian limestone and the overlying Hudson shales in all the states on the 

 line from Vermont to Alabama, and in many places it is worked for the iron. But the 

 ore is a result of the decomposition of the rocks long subsequent to the Lower Silurian 

 era (page 126). 



GENERAL OBSERVATIONS ON THE LOWER SILURIAN. 



ROCKS. 



It is a point to be noted that, during the Lower Silurian, the rocks of the 

 Continental Interior over the Mississippi Basin were chiefly limestones ; 

 and that in the Trenton period the limestones extended in great force to and 

 beyond the Appalachian protaxis. There is no evidence of their origin at 

 abyssal depths. The beds were mostly made in clear waters near the sur- 

 face, as in modern coral seas, and at moderate depths, probably not exceeding 

 a few hundred feet. Magnesian limestones prevail below the Trenton, and 

 occur to some extent within this formation; and such limestones (dolo- 

 mytes) are strong evidence of a sea-marsh condition during their origin, or 

 of shallow sea-border flats, as explained on page 133. Such an origin also 

 explains that fine trituration of all the calcareous relics, which made the 

 magnesian limestone so generally unfossiliferous. 



CLIMATE. 



No proof that a marked diversity of zones of climate prevailed over the 

 globe is observable in the fossils of the Cambrian period, or of any part of 

 the Lower Silurian era, so far as yet studied. The difference between the 

 polar regions and the parallel of 40° was probably not greater than between 

 cold temperate and warm temperate. It has been inferred that some differ- 

 ence in zonal temperature exists from the closer resemblance of fossils of 

 northwestern Scotland to those of Canada (page 572) than to those of Eng- 

 land, and the existence of the Gulf Stream of the Cambrian Atlantic is sug- 

 gested by G. P. Matthew. The following species, common in the United 

 States, and occurring at least as far south as Tennessee, have been found in 

 the strata near Lake Winnipeg: Stropho7ne7ia (Rqfinesqidna) alternata, Lep- 

 tcena {Plectamhonites) sericea (?), Madurea magna, Rapliistoma lenticulare, 

 Calymene calUcephala, MonticuUpora lycoperdon, Receptaculites Neptuni. 



The mild temperature of the Arctic waters is evident from the occurrence 

 of the following species on King William's Island, Korth Devon, and at 

 Depot Bay, in Bellot's Strait (lat. 72°, long. 94°) : MonticuUpora lycoper- 

 don, Orthoceras moniUforme H., Receptaculites Neptuni De France, Actino- 



