192 GEOLOGY. 



279. Gypsum. There is also much gypsum in the 

 rocks of the saliferous epoch of the Silurian age in New 

 York. It is found, not in layers, but in masses imbedded 

 in the rocks, from some of which it is supposed to have 

 been formed by a chemical process. The explanation of 

 the process is this : The sulphureted hydrogen issuing 

 from sulphur springs, which abound in New York, by be- 

 coming oxydized, produces sulphuric acid, and this, act- 

 ing upon the limestone (carbonate of lime), unites with 

 the lime, driving off, of course, the carbonic acid ( 60). 

 Wherever gypsum and salt are found together, as they 

 often have been, the salt, being soluble, may be carried 

 off by water, if circumstances allow of drainage, leaving 

 the insoluble gypsum behind. In many cases where 

 gypsum is found alone there was once salt in company 

 with it. 



280. The Silurian Beach. As the land formed in the 

 Azoic age was not elevated to any great height, the land 

 which was added to it during the age of Mollusks was 

 mostly added as accumulations are now made on a wide- 

 spread beach by the waves of the sea. So much was 

 this the case, that Professor Agassiz remarks that " ir. 

 the Silurian period, the world, so far as it was raised 

 above the ocean, was a beach." He speaks also of cer- 

 tain marks of water movements, usually seen on beaches, 

 which are now found in the solid rocks of this age. 

 Even ripple-marks made, not merely centuries, but thou- 

 sands of centuries ago, are to be plainly seen now in the 

 rocky strata as we divide the laminae and expose fresh 

 surfaces. So there are ridges left which are manifestly 

 remains of ancient sea-shores, formed one after another 

 as the land, with the accumulation of deposits, encroached 

 upon the water. There are many of these ridges to be 

 seen extending from the neighborhood of Lake Cham- 

 plain toward the west, marking the limits of the sea at 

 successive periods in the age. They have all the irreg- 

 ularities of line and shape that are witnessed in sea- 



