AMERICAN 387 



Wisconsin. The sea evidently deepened to the westward, for here 

 the Clinton is represented by limestones. Next came a time of 

 limestone making on a great scale (the Niagara), indicating a gen- 

 eral deepening of the water, even to the eastward. The gradual 

 nature of the change is shown by the fact that in western New 

 York the lower part of this stage is a shale, with limestone above. 

 This arrangement is beautifully displayed in the gorge of the 

 Niagara River (see p. 100), and to it is due the continued verti- 

 cally of the falls. In the East the Niagara limestone has little 

 southward extension, not occurring even in Pennsylvania, but 

 southwestward it stretches for nearly 1000 miles, to Wisconsin and 

 thence over Illinois, Iowa, Missouri, and western Tennessee. It 

 recurs in the Black Hills of South Dakota, and in Nevada is 

 represented by the summit of a thick mass of limestone, which 

 extends upward unbrokenly from the Trenton. The Niagara 

 limestone is very largely made up of corals and in some places 

 the ancient reefs may be identified. In the narrow channels to 

 the northeastward, connected with the Gulf of St. Lawrence, 

 were laid down the Niagara rocks which now are found in New 

 England, New Brunswick, and Nova Scotia. 



The next change was the separation, along the northern por- 

 tion of the interior sea, of a series of salt-water lagoons, in which 

 were deposited red marls and shales with gypsum and rock salt 

 {Salina), from which are obtained the brines of New York, On- 

 tario, and Ohio. These rocks are thickest in New York and 

 Pennsylvania, thinning to the south and west. In part contem- 

 poraneous with the Salina is the Water-lime, so called because 

 it is a hydraulic limestone > and employed for making cement. It 

 is a thinly bedded argillaceous and magnesian limestone, in places 

 enclosing masses of gypsum. The Water-lime has much the same 

 distribution as the Salina, but is thickest where the latter is thin. 



A renewed change of level, this time a depression, brought 

 the sea in again where the salt lagoons had been, extending it 

 farther to the eastward at the same time. In this clearer and 

 deeper sea were laid down the limestones of the Lower Helderberg 

 series. The old channel between the Appalachian and the north- 



