40 



At the end of the Ordovician or beginning of the Upper Silurian age, 

 the Interior Paleozoic Sea liad greatly diminished in area. A broad belt 

 of land had l)een added to the southern border of the old Laurentian crest, 

 especially over \vhat is now Wisconsin and a portion of northern 

 Illinois: while, extending from what is now Labrador down to Georgia. 

 was another broad belt, following the general trend of the present Alle- 

 ghany mountains. By the raising of se\eral large islands above its sur- 

 face at the time of the Cincinnati Uplift, aided by the broad belt of 

 shallowly submerged land already noted, the area of the Interior Sea was 

 still further diminished and to that portion covering what is now the 

 northeastern part of Indiana and the greater part of Ohio, West Virginia, 

 New York and Pennsylvania, the name of "Eastern Interior Sea" is given. 

 This was simply a great bay or eastward extension of a greater "Central 

 Interior Sea" which, at that period, covered most of Indiana, southern 

 Michigan, Illinois and a large portion of the present United States west of 

 the Mississippi River. The most northeastern limits of the Eastern In- 

 terior Sea were the present sites of Albany and Troy, New York. Tlie 

 rock-making material which Avas deposited on the floor of both it and the 

 Central Interior Sea Avas derived in part from tlie land along their bor- 

 ders, but mainly from the limey secretions of the life within their waters. 

 The dry land draining into them was small in area and hence there Avere 

 only small streams for the supply of sediments. Yet, in the course of 

 countless years, suflicient material was deposited to form the thick layer 

 of Niagara limestone Avhich noAV forms the surface rock over much of 

 northern and eastern Indiana. 



The epochs of the Upper Silurian age, as represented in Indiana, are 

 three in number, viz., the Clinton, the Niagara and the Water Lime, or 

 LoAver Helderberg. Kach is represented by its characteristic rocks, bear- 

 ing the peculiar fossils of its time. The Clinton epoch is represented iu 

 the State by a close-grained, salmon-colored limestone, varying iu thick- 

 ness from a few inches only to altout seven feet. It outcrops in a very 

 narroAv strip along the Avestern edge of the area of the Hudson River 

 limestone, already mentioned as the oldest rock in Indiana, and overlies 

 that formation beneath the surface of at least the ea,stern third of the 

 State. It has no economic importance and serves only as a line of de- 

 marcation separating the older Silurian rocks from those great beds of 

 Niagara limestone Avhich Avere afterward laid doAvn in the Upper Silurian 

 seas. 



