43 



where have been found numeroiis fine examples of its most characteristic 

 fossils— gigantic crustaceans, two feet or more in length, closely related 

 to the king crabs of the present seas. Over the extensive mud flats of 

 the closing period of Upper Silurian time they were the undoubted rulers, 

 while in the nearby waters sported descendeuts of those mail-clad fishes 

 which first appeared in the Trenton period of the Lower Silurian era. 



The Lower Helderberg represents the final epoch of Upper Silurian 

 time. In Indiana its rocks form a buff to gray cherty limestone, 25 to 250 

 feet in thickness and often irregular and uneven in its bedding. It di- 

 rectly overlies the Niagara limestone where the Water Lime is absent. 

 Outcrops occur at Logansport and other points to the northwest, and 

 drill holes sunk for oil and gas shoAV that it probably forms a portion of 

 the surface rock beneath the deep drift-covered area of the northern third 

 of the State. 



The advance in life during the Upper Silurian era was not propor- 

 tionally as great as that of the preceding age. The earliest of Arachnids, 

 the scorpions, came to be, their first remains being in the Water Lime, 

 showing that they were neighbors of the giant Eurypterid ci'ustaeeans. 

 Cockroaches and progenitors of dragonfiies were also present, but re- 

 mains of other terrestrial forms are few or lacking. Among marine 

 invertebrates, Cephalopods reached the acme of their development, the 

 gigantic Orthoceratites of this group, whose remains are so common 

 in the Niagara limestones of Wabash and adjoining counties, being 

 worthy of especial mention. 



We have seen that liy the beginning of the Devonian Age or Era, 

 which succeeded that of the Upper Silurian, the waters of that great 

 bay known as the Eastern Interior Sea, had become farther separated 

 from those of the Central Interior Sea by the uprising of the Niagara 

 limestone area of eastern Indiana and western Ohio, and also by the 

 deposition along the margin of this formation of the sediment comprising 

 the Water Lime and Lower Helderberg limestones. A probable connec- 

 tion still existed between the waters of these two basins across the 

 broken or interiiipted strip connecting the main body of Niagara lime- 

 stone in eastern Indiana with the main land area of the same formation 

 in northwestern Indiana and northern Illinois. 



The Devonian rocks of Indiana may be roughly classed as represent- 

 ing two great epochs, the Corniferous and the Genesee, the former being 

 represented by beds of more or less pure limestone, ranging up to 55 



