190 GEOLOGY. 



ited, the azoic rocks, A, would be underneath the whole. 

 The Silurian system ends in the Helderberg series, and 

 the Devonian lies between this and the beginning of the 

 Carboniferous system, T. The old red sandstone, S, is so 

 prominent in the Devonian system in England and Scot- 

 land that its name is there very commonly applied to the 

 whole system. 



276. Geographical Distribution. The chief part of the 

 rocks of this system that come to the surface in the 

 United States lie along south of the continent as it was 

 begun in the Azoic age, as represented in 267. They 

 are in the great lake region, and extend along in a broad 

 belt west from Lake Michigan. A long strip of this sys- 

 tem stretches along on the east of the great Appalachian 

 coal-field, that extends from Pennsylvania down into Ala- 

 bama. The Silurian rocks are found in the west of En- 

 gland, where, as I have before said, they were first suc- 

 cessfully examined ; also in Belgium, Germany, Norway, 

 Sweden, and in Russia, in the neighborhood of St. Peters- 

 burg. 



277. Copper of the Silurian Rocks. In the neighbor- 

 hood of Lake Superior trap rocks were thrust upward 

 among the Silurian rocks, and native copper is found 

 there in great abundance in veins, in both kinds of rocks 

 where they came together. It is supposed that the cop- 

 per was produced from ores in the azoic rocks lying be- 

 low, the trap, as it came up in its molten state, smelting 

 the ores as if in a vast furnace, and forcing the pure metal 

 upward, which, in its melted state, passed into openings 

 and crevices, making veins in all directions. 



278. Silurian Salt. Most of the salt-springs in this 

 country issue from Silurian rocks. The extensive accu- 

 mulation of salt in the State of New York Professor 

 Dana accounts for in the following manner. He sup- 

 poses that when the saliferous (salt-bearing) beds were 

 laid down there was in that quarter a large shallow ba- 

 sin, or series of basins, with limestone, such as we have 



