AGE OF MOLLUSKS. 191 



at Niagara Falls, for a bottom. Into these basins the 

 salt water of the ocean was admitted, in some way, in- 

 termittingly. This might be from the occasional break- 

 ing away here and there of the barriers which bounded 

 in these basins from the sea, or simply from the changes 

 of the tide. The result, you see, then, would be some- 

 what like that which we have in the artificial production 

 of salt from evaporation of the waters of the ocean, as 

 noticed in 358, Part II. By evaporation over this im- 

 mense area of the basins the salt of the sea-water would 

 be deposited, mingled with the mud, which would be at 

 the same time settling as sediment. This would go 011 

 rapidly whenever the water was very low, especially 

 with the warmth of climate which prevailed in that age. 

 And such accumulations of salt going on for a long pe- 

 riod (for this salt-making, like the coal-making, did oc- 

 cupy a long time in the world's geological history) would 

 lay up in the rocks into which this mud would change 

 by solidification a vast amount of salt, such as we now 

 find in the Silurian deposits in New York. Mr. Dana 

 states that he once saw in a small coral island of the 

 Pacific a process similar to this actually going on. It 

 was an island with a lagoon in it, having no free com- 

 munication with the ocean. The waters became ex- 

 tremely salt in the hot, dry season, and fresh again in 

 the rainy months. There was a mud deposited in the 

 bottom of the lagoon from the washing of the coral 

 rocks that bounded it. This calcareous mud, if solidi- 

 fied, would make a saliferous rock, like the rocks of this 

 character in the Silurian system of New York. The salt 

 is obtained from these rocks in New York by evapora- 

 ting the solution, or brine, made by the water which 

 gains access to the strata. At the great salt-works in 

 Salina and Syracuse this brine is collected by borings, 

 which sometimes extend down over 300 feet. From 35 

 to 45 gallons furnish a bushel of salt, while it requires 

 nearly ten times this quantity of sea-water to furnish this 

 amount. 



