522 HISTORICAL GEOLOGY. 



From western China, Richthofen has reported Orthis calligramma, Leptoena {Plectam- 

 honites) sericea, Spirifer radiatus, Atrypa reticularis, Favosites fibrosus, Heliolites inter- 

 stinctiis, Halysites catemdatus, etc. In southern Australia, in Victoria, Lower Silurian 

 beds, made 35,000' thick by Mr. Selwyn, have afforded various Graptolites of the common 

 Lower Silurian genera. 



ECONOMICAL PRODUCTS OF THE LOWER SILURIAN FORMATIONS. 



Lead Ore, Galena. — The Galena limestone of Wisconsin and the adjoin- 

 ing states on the south and west derives its name from the valuable lead 

 deposits which it contains. Similar deposits occur in the Lower Silurian 

 limestones of Missouri (though not at present profitable like those of the Cam- 

 brian and Subcarboniferous limestones of that state) and also in Arkansas. 

 The large Joplin mines of Missouri are in the Subcarboniferous. On these 

 deposits see under "Veins," page 342. None of them, as there stated, are of 

 Lower Silurian origin, but of some later, unascertained date. 



Mineral Oil and Gas. — Mineral oil and gas come from the decomposition 

 of animal or vegetable materials, "when buried and under close confinement 

 from the atmosphere. The Trenton limestone and the Utica and Hudson 

 shales have long been known to afford mineral oil, especially since the early 

 reports on the subject by T. S. Hunt, who rightly referred these substances 

 to organic materials buried in the limestone or shale at the time of their 

 formation (1861, 1866). The black color of the Utica shale is due to car- 

 bonaceous substances, and oil is easily obtained by heating ; and in Colling- 

 wood, Canada, there were formerly works for the purpose, 30 to 36 tons of 

 shale yielding 250 gallons of crude oil (at a cost of about 14 cents per gallon) 

 — an amount corresponding to about 3 per cent of the rock (Hunt). At 

 Manitoulin Islands, also, petroleum was early procured by boring. Whitney 

 obtained 21 per cent from the shale of Savannah, 111. ; 11 to 16 per cent 

 from that of Dubuque ; and 12 to 14 per cent from that of Herkimer County, 

 N.Y. The oil has been found in Orthocerata at Pakenham, Canada, and in 

 fossil Corals at Watertown, IST.Y. 



The distillation process was long since thrown aside in consequence of 

 the free supplies of the liquid oil through Artesian borings; and among the 

 productive rocks are soi^e of the Lower Silurian. The idea, now fully sub- 

 stantiated, that the oil and gas are usually to be obtained along anticlinals, 

 was announced in 1861 by T. S. Hunt, and independently by E. B. Andrews. 



In Ohio and eastern Indiana the Trenton limestone affords both oil and 

 gas abundantly, but chiefly the latter. The region is within the underground 

 range of the Cincinnati anticline, and the principal Ohio localities are at 

 and near Findlay, 150 miles north of Cincinnati, on the axial part of a portion 

 of the anticline, where it has a local upward bulge or bend; and to this 

 upward bulge in the axis the Findlay region appears to owe its gas-confining 

 power. The borings descend 1100 to 1200 feet to the Trenton limestone, and 

 only 15 to 25 feet, or, in some parts, 50 feet, into the rock, a greater depth 

 usually being only sparingly productive. The Findlay wells yielded, in 1886, 



