36 



primary rocks whose molten sands rose lirst above tlie Ijoiling floods and 

 cooled and criisted into a chaotic continent. For Arclnean ±ime com- 

 prised those millions of years which elapsed while the crust of the earth 

 Avas cooling down to a i oint where life was possible. 



The Laurentiau rocli^s are thus devoid of fossils or contain only tlie 

 remains of the simplest aquatic forms. In North America they com- 

 prise the surface of a vast, V-sliaped area of 2,000,000 or more square 

 miles which lies, filled with wild lakes, pine clad, rugged, almost impass- 

 able, spread in savage sleep from Labrador to the Arctic Ocean. This area 

 embodies the general form of the North American continent and was the 

 nucleus of all the land which was afterward added to it. From these old 

 Laiu'entian rocks came the debris and sediment which was laid down in 

 the bed of a shallow ocean to form the first rocks comprising the surface 

 of what is now '"Indiana." 



At the close of the Azoic or Lifeless oeon, during wliicli the Laureutian 

 rocks were formed, the Paleozoic or "JEon of Ancient Life" was ushered 

 in. At its beginning the entire area of what is now known as Indiana 

 was covered by a broad ocean which stretched far away to the southwest, 

 while to the north and northeast it extended beyond the present sites of 

 the Great Lakes. This ocean is known to geologists as the '"Interior 

 Paleozoic Sea." Into it was carried the sediment derived from the erosion 

 and destruction of the old Laureutian roclvS by water and ail", which 

 agencies then, as now, were ever at work. The Potsdam sandstone of the 

 Cambrian era, which probably underlies tlie Trenton limestone of the 

 Lower Silurian beneath the greater portion, if not all, of Indiana, was 

 one of the first strata to be laid down in this sea. But as none of the 

 surface of Indiana is represented by the Potsdam stone, it will be passed 

 with this mere mention. 



Following the Cambrian came the second grand sub-division of Paleo- 

 zoic Time, the so-called Lower Silurian or Ordovician Age. At its 

 beginning the sea covering Indiana and the area to the north and east 

 was of course more shallow, as 1,000 feet or more of Potsdam sandstone 

 had been deposited on its floor. The first great stratum of Ordovician 

 rock to be laid down in this sea which is of interest to us was the 

 Trenton limestone, which, during the past two decades, has become so 

 noted in Indiana as the source of natural gas and crude petroleum. 



It is a well known geological fact that most, if not all, limestones owe 

 their origin to tlie pre.'^ence of minute organisms in the water in which 



