PAPERS ON BIOLOGY AND AGRICULTURE 93 



gannister and used in the manufactuiT of fii-e-brick. Such 

 mines are to be seen near Cauble School, Sect. 1. T. 14 S., 

 R. 2 \V.. and near Elco. 



Below these shales are the sandstones and limestones of 

 the Middle Devonian. The most conspicuous of these is the 

 Clear Creek chert refeiTed by Savage to the Ulsterian or 

 Middle Devonian series. This has a total thickness of 300 

 feet and consists in large part of layers of more or less de- 

 composed chert or in places in the upper portion of alter- 

 nating layers of chert and limestone. From this formation 

 most of the silicon in Union and Alexander Counties is ob- 

 tained, while poitions by decomposition form a white plastic 

 clay or "kaolin." A silicon mine is now being operated at 

 Delta. A quariy near Tamms exposes a vertical cliff of 180 

 feet which is largely composed of chert. 



The Lower Devonian is represented by some limestones 

 of the Helderbergian series seen within our area only in the 

 river cliff between McClure and Gale. They are in contact 

 with other hmestones of Silurian age called the Sexton 

 Creek formation. This consists in the lower part of a hard 

 gray limestone in layers 4 to 8 inches thick separated from 

 one another by 2 to 4 inch bands of chert. On the top is a 

 bed of hard, fine-grained pink or mottled limestone in rather 

 thick layers. It is well shown in the east bluff of the river, 

 one and a half miles east of McClure and also along the 

 valley of Sexton Creek. It varies in thickness from 16 to 79 

 feet. 



In the northern part of the county there seem to be no 

 rock exposures of formations older than the Sexton Creek 

 limestones, but near Thebes are outcrops of Girardeau lime- 

 stone. This Savage (3) places lower in the Silurian and 

 refers it and the overlying Silurian limestones to the Alex- 

 andrian series because of their exposure in Alexander 

 County. It is a dark, fine-grained, hard, brittle rock and has 

 a total thickness of about 35 feet. 



The oldest rocks in the countj' belong to the Ordovician 

 and are the Thebes sandstone and the Kimmswick limestone. 

 The former, a reddish brown arenaceous rock some 75 feet 

 in thickness, is favorably exposed just south of the village 



