37 



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

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

 —the polyps and bryozoans of the ancient Silurian seas. In mitold num- 

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

 cent, of the unmodified Trenton roclv, 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 

 were 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 ciu-rents. Rivers from the older Cambrian rocks 

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

 ocean floor. Within these beds of sediment Ijoth plants and animals 

 found a grave — their bodies in vast numbers being buried Ijeneath 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 oxj'- 

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

 ment was. by the agency of the sea Avater, cemented and consolidated into 

 limestone. In this manner that great layer of Trenton rock w^hich 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, either underlying oi' forming a portion of the surface of 

 the State. 



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

 Trenton limestone, with its enclosed accumulations of luidecayed plants 

 and animals, became turl)id, and instead of calcareous sediment, depos- 

 ited mud and clayey sediment in thick beds on top of the limestone strata. 

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

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

 thickness, which thus effectuallv sealed the Trenton limestones and so 



