18 THE GEOLOGIST'S TRAVELING HAND -BOOK. 



4 a, Trenton Limestond Next in ascending order occurs the 4 a. Trenton 

 limestone which, in the Northwestern States, is divided into the Buff limestone 

 and the Blue limestone. In Wisconsin there are two buff and two blue beds 

 alternathig. They are undoubtedly the same as the well known Chazy, Birdseye, 

 Black River and Trenton limestones of New York and other Eastern States. They 

 are known in the "West wherever the exposures reach to the upper sandstone. 



The upper member of the 4 a. Trenton limestone, in South Western Wisconsin 

 and the adjoining parts of Illinois and Iowa, is the very important GALENA or lead- 

 producing limestone, which has no exact representation in the Eastern States. It 

 is a light gray or yellowish-gray, heavy-bedded rock. It is compact, minutely 

 crystalline throughout, often with small cavities lined with crystals of brown 

 spar, and the whole thickness of the formation is 250 feet. The Galena or lead 

 ore contains 13.4 per cent, of sulphur and 86.6 per cent, of lead, and is found 

 in heavy bodies in crevices in this Galena dolomite or magnesian limestone. 

 Prof. J. D. Whitney, in his admirable report on the geology of the lead region of 

 Southwestern Wisconsin, has proved that these lead deposits must have been 

 introduced into the fissures by precipitation from above. The lead mines of 

 Missouri are chiefly in the Lower Magnesian limestone. 



In Wisconsin, a very noticeable feature of the Trenton limestone is its 

 marked division into the two parts before mentioned. One, which is the lower 

 half, is very heavy bedded, in layers of two or three feet thick, known as the glass- 

 rock, and the other thin bedded, in layers of two or three inches. There is always 

 a stratum of carbonaceous shale from a quarter of an inch to a foot or more in 

 thickness, which separates the blue or Trenton from the thin bedded Galena 

 limestone above it. 



Professor R. D. Irving describes the Galena limestone as almost invariably a 

 very compact, hard, crystalline rock, of a yellowish-gray color, with numerous 

 small cavities filled with a softer material, or lined with crystals of calcite. The 

 upper portion is thick-bedded and free from flints, the layers being from one to 

 four feet thick, while the lower portion almost invariably consists of several feet 

 of layers from one to two inches thick. Good exposures of parts of the Galena 

 limestone are frequently to be met with. It may be seen in cliffs and ledges, on 

 nearly all the streams in the lead-region, where it weathers irregularly, leaving the 

 surface full of small cavities, due to the removal of its softer parts. The formation 

 contains masses of flint in layers, or in irregular pieces, which are principally 

 confined to the middle and lower parts of the formation, although not entirely 

 absent from any part. 



In the interior valleys of Pennsylvania, as for example, in Sinking Valley, 

 Blair Co., considerable quantities of zinc-ore, and some galena, have been found in 

 the Trenton limestone group, which is there at least 1,000 feet thick. The lead- 

 mines of Wythe Co., Virginia, are at the .same, or at a somewhat lower horizon. 

 The zinc mines near Bethlehem, Pennsylvania, and near Landisville, Lancaster 

 Co., are nearly of the same geological age. Isolated crystals or small masses of 

 galena occur in crevices in the limestone beds of this age throughout the entire 

 range of the great valley from Newburgh, on the Hudson, to Chattanooga, in 

 Tennessee. The limestones hi this valley, which are the Auroral limestones of 

 Rogers', are, by some geologists, referred to an older series. See, in this connection, 

 the foot note on page 21. 



