Handbook of Paleontology 257 



part of the structure being known as the Cincinnati dome, 

 the southern part as the Nashville dome. 



Petroleum and natural gas have been found in all of 

 the fossiliferous rocks from the Ordovician through the 

 Tertiary. Some of the richest pools have been found in 

 the Ordovician and Devonian. The Ordovician (Mo- 

 hawkian) limestones of Ohio have yielded large quanti- 

 ties of oil and gas. Among other economic products of 

 the period are the slates of the Appalachian region, espe- 

 cially those of Vermont and Pennsylvania (Upper Ordo- 

 vician). Because of the wide distribution of the lime- 

 stones there are important marble, limestone and cement 

 industries. Lead deposits occur in the Galena formation 

 (post-Trenton age and probably later) of the middle 

 west. The deposits of zinc in Wisconsin are in pre- 

 Trenton (Mohawkian) rocks. 



Life. So far as fossils are concerned in the separa- 

 tion of these early Paleozoic systems it may be said that 

 true graptolites, such ostracods as Leperditia and true 

 Orthidae are first seen in the Upper Canadian ; whereas 

 such other brachiopods as Orthis, Dalmanella, Platystro- 

 phia and Camarotoechia and such trilobites as Illaenus, 

 Isotelus, Calyttiene etc., certain groups of corals, true 

 bryozoans, pelecypods, true ostracods and typical crinoids 

 are unknown beneath the base of the Ordovician. 



In the Canadian rocks of the St Lawrence province 

 the fossils are chiefly graptolites belonging to the Atlantic 

 realm and similar or identical with species of Great Brit- 

 ain, Norway and Sweden. These are found in the dark 

 shales characteristic of that area and also in similar 

 shales of like age farther south in Vermont, New York 

 and Pennsylvania (Deepkill). Among the characteristic 

 graptolite genera of these shales are Tetragraptus, Phyl- 



