37 



the limestone was formed. The animals from whose remains the Trenton 

 limestone was, for the most part, derived, were probabljf very low forms 

 —the polyps and bryozoaus of the ancient Silurian seas. In untold num- 

 bers they existed, and the carbonate of lime, which makes up 80 per 

 cent, of the unmodified Trenton rock, is largely the remains of their 

 secretions and incrustations. Associated with these lower forms were 

 myriads of higher ones— crinoids, brachiopods, trilobites, gastropods and 

 even fishes. The presence of such swarms of animal life made necessary 

 the existence of an abundance of plants; since the plant must ever pre- 

 cede the animal and gather for the latter the energy, and form for it the 

 food— the living protoplasm— necessary to its existence. These plants 

 wei-e mostly marine algae or seaweeds and fucoids, though doubtless 

 many other forms existed of which no remains have been preserved in 

 the rocks of that age. 



The Trenton limestones were evidently formed in rather clear waters, 

 at moderate depths. Near the bottoms of these shallow seas great beds of 

 calcareous sediment were gradually collected, and were swept to and fro 

 by the tides and currents. Rivers from the older Cambrian rocks 

 brought down their eroded particles and added to the thickness of the 

 ocean fioor. Within these beds of sediment Itoth plants and animals 

 found a grave— their bodies in vast numbers being buried beneatli the 

 slowly accumulating deposits of centuries. Once buried in such deposits, 

 they did not decay, as do animals on land, because by the waters above 

 and the calcareous ooze around them, they were shut off from free oxy- 

 gen, which is the chief agent in decay. Gradually this ooze or fine sedi- 

 ment Avas, by the agency of the sea water, cemented and consolidated into 

 limestone. In this manner that great layer of Trenton rock which under- 

 lies at variable depths the whole of Indiana, was formed. From it has 

 been derived, directly or indirectly, more wealth than from any other 

 one formation, eitlier underlying or forming a portion of the surface of 

 the State. 



In time the waters of the ocean containing this vast stratum of 

 Trenton limestone, with its enclosed accumulations of undecayed plants 

 and animals, became turbid, and instead of calcareous sediment, depos- 

 ited mud and clayey sediment in thielv l3eds on top of the limestone strata. 

 These deposits of mud and silt were afterward, by later deposits, com- 

 pressed into the fine-grained, impervious Utica shale, 100 to 300 feet in 

 thickness, which thus effectually sealed the Ti'enton limestones and so 



