THE STORY OF OHIO'S ROCKS 



17 



is much less at present. All this oil -and oil from other rocks as well-is derived from the bodies 

 of animals and plants, by complex changes through long periods of time in the depths of the earth. 



As Ordovician time came to a close, the waters of the sea retreated from Ohio but they 

 soon returned to cover the state again in Silurian time. 



SILURIAN TIME: THE RISE OF THE CORALS 



In the early part of the Silurian period Ohio was 

 dry land. The advance of the Silurian seas, both from 

 the south and the north, was slow, and in the western 

 part of the state lovv islands existed from time to time 

 (see fig. 15). In Middle Silurian time, however, the 



• • • • j» > 



• • • • • •. _•• • 



* • • * • • ••• *\ * . • 



Fig. 15 Middle Silurian lands and seas 



seas reached Ohio and soon covered most of it. They 

 were sometimes muddy, sometimes clear, resulting 

 in the accumulation of shales, limestones, and dolo- 

 mites in our Middle Silurian section (fig. 16). 



Ohio generally ranks second among the states 

 in the production of limestone and dolomite. The 

 Brassfield formation supplies some of the high calcium 

 limestone. Most of the dolomite quarried in Ohio is 

 of Niagaran age. Some of the stone is used in its 

 natural condition, some is heated in great kilns to 

 convert it into lime. Thus we have both high calcium 

 lime, from limestone, and magnesian lime, from 

 dolomite. Ohio is the nation's greatest lime producer. 

 The many quarries of the western half of the state 

 produce rock for building stone, roads, railroad 

 ballast, concrete aggregate, blast furnace flux, 

 agricultural lime, Portland cement, bonding materials 

 for silica brick, ceramic bodies and glazes, dolomite 

 refractories, water softening, sugar refining, and a 

 dozen other uses. 



Bass Islands group 550' 



Niagara group 250' 

 Guelph 



Cedarvtlle 



/ / T, 



Fig. 16 Silurian rocks of Ohio 



