230 



REPORT OF NATIONAL MUSEUM, 1899. 



bauxites of the Southern United States show considerable variety in 

 physical appearance, though generally having a pronounced pisolitic 

 structure. (See Specimens Nos. 63335, 66576, 6657T, and 66578, 

 U.S.N.M. , from Floyd and Bartow counties, Georgia; also fig. 5, p. 229.) 



The individual pisolites vary in size from a fraction of a millimeter to 3 or 4 centi- 

 meters in diameter, although most commonly the diameter is from 3 to 5 millimeters. 

 The matrix in which they are embedded is generally more compact and also lighter 

 in color. The larger pisolites are composed of numerous concentric shells, separated 

 by less compact substance or even open cavities, and their interior portions readily 

 crumble to a soft powder. 



In thin sections the ore is seen to be made up of amorphous flocculent grains, and 

 the various structures which it exhibits are produced by the arrangement and degree 

 of compactness of these grains. The matrix in which the pisolites are imbedded 

 may be composed of this flocculent material segregated in an irregularly globular 

 form or in compact oolites, with sharply-defined outlines. Or both forms may be 

 present, the compact oolites being embedded in a matrix composed of the less defi- 

 nite bodies. In some cases the interstices between the oolites are filled either wholly 

 or in part with silica, apparently a secondary deposition. 



The pisolites also show considerable diversity in structure. In some cases they 

 are composed of exactly the same flocculent grains as the surrounding matrix, 

 from which they are separated by a thin shell of slightly denser material. This 

 sometimes shows a number of sharply-defined concentric rings, and is then dis- 

 tinctly separated from the matrix and the interior portion of the pisolite. The latter 

 is also sometimes composed of imperfectly defined globular masses, and in other 

 cases of compact, uniform, and but slightly granular substance. It is always filled 

 with cracks, which are regularly radial and concentric, in proportion as the interior 

 substance has a uniform texture. Branching from the larger cracks, which, as a rule, 

 are partially filled with quartz, very minute cracks penetrate the intervening por- 

 tions. Thus the pisolites appear to have lost a portion of their substance, so that it 

 no longer fills the space within the outer shell, but has shrunk and formed the radial 

 cracks. No analyses have been made of the different portions of the pisolites or of 

 the pisolites and matrix separately, and it is impossible to say whether any differ- 

 ences in chemical composition exist. It may be that some soluble constituent has 

 been removed from the interior of the pisolites, but it is more probable that the 

 shrinking observed is due wholly to desiccation. 



Scattered throughout the ground-mass are occasional fragments of pisolites, whose 

 irregular outlines have been covered to varying depths by a deposit of the same 

 material as forms the concentric shells, and thus have been restored to spherical or 

 oval forms. 



Composition. The following tables will serve to show the wide range 

 of composition of bauxites from various sources: 



Composition of bauxites from various localities. 



