350 HISTORICAL GEOLOGY 



rapidly removed by erosion. The sand and finer sediments 

 sifted from the gravel were carried farther westward, forming 

 the Medina sandstone. As the high lands were worn down, 

 the rivers became less active, and less gravel was strewn along 



the front of the 

 mountains. As 



FIG. 356. Diagram showing the relation of the the zone of gravel 

 Silurian limestone in the Mississippi Valley to the accumulation be- 

 conglomerate, sandstone, and shale in New York. 



came narrower, 



the zone of sand deposition encroached upon it, and it thus 

 happened that the Medina sands extended continually farther 

 and farther eastward until they came to lie partly upon the 

 Oneida beds (Fig. 356). 



The Clinton iron formation. In the more remote parts of 

 the interior this rejuvenation of the New England region 

 seems to have exerted no influence. In Illinois, for example, 

 the first Silurian beds were of shale and limestone, and the 

 deposition continued without change in the character of the 

 sediments until the latter part of the period. Between the 

 sandy coastal plain and this clear, open sea there was an 

 irregular belt over which sediments rich in compounds of iron 

 were deposited on a large scale. This phase of the Silurian 

 rocks has been named the Clinton formation. The iron ore 

 is usually of the red variety or hematite; in some places, 

 where massive beds several feet in thickness are found, pro- 

 ductive iron mines are located. The microscope shows that 

 some of this ore has the structure of limestone, that is, the 

 rock is composed of bits of shells, corals, etc., but the material 

 is largely iron oxide instead of lime carbonate. Students of 

 the subject are not yet agreed as to the exact conditions under 

 which these unusual deposits were made, but there seems good 

 reason to believe that the sediments were laid down in shallow 

 water not far from land. 



In the process of smelting iron ore it is mixed with limestone 

 and coke, and when the mixture is heated in the furnace, the iron 

 is released from the ore and flows out into the molds. At Bir- 



