THE NONMETALLIC MINEBALS. 217 



As the material is worth but from $1 to $2 a ton delivered at 

 Trenton, it may be readily understood that transportation is a rather 

 serious item to be considered in developing home resources. 



According to Mr. R. T. Hill, nodules of black flint occur in enormous 

 quantities in the chalky limestones the Caprina limestones of Texas. 

 Numerous localities are mentioned, the most accessible being near 

 Austin, on the banks of the Colorado River. 



BUHRSTONE, or burrstone, is the name given to a variety of 

 chalcedonic silica, quite cavernous, and of a white to gray or slightly 

 yellowish color. The cavernous structure is frequently due to the 

 dissolving out of calcareous fossils. The rock is of chemical origin 

 that is, results from the precipitation of silica from solution, and pre- 

 sumably through the action of organic matter. In France the material 

 occurs alternating with other unaltered Tertiary strata in the Paris 

 basin (Specimen No. 36140, U.S.N.M.). It is also reported in Eocene 

 strata in South America, and in Burke and Screven counties along the 

 Savannah River in southern Georgia in the United States (Specimen 

 No. 36051, U.S.N.M.). The toughness of the rock, together with the 

 numerous cavities, impart a sharp cutting power such as renders them 

 admirably adapted for millstones, and in years past material for this 

 purpose has been sent out from French sources all over the civilized 

 world. 



TRIPOLI is the commercial name given to a peculiar porous rock 

 regarded as a decomposed chert associated with the Lower Car- 

 boniferous limestones of southwest Missouri (Specimen No. 55028, 

 U.S.N.M.). The rock is of a white cream or slight pink cast, fine 

 grained and homogeneous, with a distinct gritty feel, and, though soft, 

 sufficiently tenacious to permit of its being used in the form of thin 

 disks of considerable size for filtering purposes (Specimen No. 62044, 

 U.S.N.M.). According to Hovey 1 the deposit is known to underlie 

 between 80 and 100 acres of land, in the form of a rude ellipse, with its 

 longer diameter approximately north and south. From numerous 

 prospect holes and borings it has been shown to have an average thick- 

 ness of 15 feet, the main quarry of the present company showing a 

 thickness of 8 feet. The following section is given from a well sunk 

 in the northern part of the area: 



Feet. 

 Earth to 4 



Tripoli 4 20 



Stiff red clay 20 21% 



Mixed chert, clay, and ochre 21 40 



Cherty limestone 40 93 



Cherty limestone bearing galena 93 103 



Limestone 103 128 



Limestone bearing sphalerite and galena 128 136 



Soft magnesian limestone 136 173 



1 Scientific American Supplement, July 28, 1894, p. 15487. 



