HORIZONS OF PHOSPHATE ROCK IN TENNESSEE 15 



bend, in Hickman. It rests on the Hudson. Westerly from this the Niagara 

 limestone soon wedges in and separates the two. Mined for the most part like 

 stone coal, by a system of tunnels and underground rooms. 



The imbedded phosphate concretions of the Maury Green shale make the fourth 

 horizon. This is sub-Carboniferous, and lies at the base of that division and right 

 above the Black shale. If the balls of phosphatic rock were mined in large quan- 

 tities, it would be mostly by tunneling. They have been used but little, having 

 been thrown out of consideration by the other abundant and superior phosphates. 



Perry County phosphate, white and variegated in color, occurs in valleys in 

 Perry county and belongs to no particular geological horizon, as its masses have 

 been deposited from water on the rocks of several formations. Sometimes the 

 Perry County phosphate presents itself in handsome marble-like pieces, not mixed 

 with foreign matter ; then again it is a medley of white mineral with broken chert. 

 It occurs in large quantity and has been industriously quarried for market. 



The age of the Mount Pleasant phosphate formation is easily shown to be Tren- 

 ton or Nashville of the Lower Silurian. Its bed or the phosphate limestone from 

 which it comes rests on what is known in Tennessee as the Orthis bed, so named 

 for the reason that it is almost wholly composed of the valves of Orthis testudinaria. 



The rock yielding the Mount Pleasant phosphate I have named " Capitol lime- 

 stone," for the reason that it supplied the rock used .in building the Tennessee 

 capitol. It is a granular, bluish gray, current-formed rock, 25 feet thick, lami- 

 nated by thin seams rich in phosphate alternating with seams of rock less so and 

 lighter in color. The lamination is best seen on weathered surfaces. Its grains 

 are mostly comminuted shells and corals. Often the rock is very rich in minute 

 fossils (Cyclorse, Conodonts, etcetera). After the leaching, more or less perfectly, of 

 the limestone there remained the available phosphate from 2 to 6 feet in thickness, 

 rarely reaching 8 or 10 feet, and yielding from 70 to 82 per cent of calcium phos- 

 phate. It underlies some 6,000 or 7,000 acres, as estimated, is only covered by the 

 soil, and is worked as an open quarry. 



When the phosphatic limestone is covered by strata of rock, leaching does not 

 take place, or but imperfectly : hence the leached product is found on the out- 

 crop and covered only by the soil. 



The Hudson phosphate comes from a phosphatic limestone which is much like 

 the last named and like it suffers leaching. The phosphate is mined in open ex- 

 cavations. It ranges from 100 to 200 feet above the Mount Pleasant rock ; con- 

 tains Cyclonema bilix, Pterinea demissa, large Orthis lynx, O. occidentalism Stropho- 

 mena alternata, and minute shells and Conodonts. 



Snow Creek phosphate is not a product of leaching, but a bed of phosphate. It 

 is a dark rock, substantially, but weathers superficially to a yellowish gray, and 

 then resembles sandstone, for which it has been mistaken. It contains the bones 

 of large fishes, and also, like the other beds below it, multitudes of the shells of 

 Cyclorse, Conodonts, etcetera. 



Concretions of the Maury Green shales occur in places, crowded together in 

 layers 18 to 20 inches in thickness. When rounded they look like so many closely 

 packed, moderately sized cannon balls. They are, however, generally loosely 

 scattered through the shale. They are wonderfully persistent, and can be found 

 almost always in place above the Black shale, though usually of small size.* 



* For information as to the economic relations of the phosphates, see " Elements of the Geology 

 of Tennessee," pp. 208-215. 



