June, 1906. New Forms of Concretions — Nichols. 37 



large dilution of the solvent when removed from the immediate 

 vicinity of the decaying root or leaf which is the 1 ource of its supply. 

 Such small limonite-sand concretions forming near the surface 

 of semi-fixed dunes are, therefore, due to an action of vegetation upon 

 the limonite coatings of the sand grains of the dune, an origin not 

 unlike that of the bog and pond limonites. 



LIMONITE GEODES, MUSCOGEE, INDIAN TERRITORY 



A series of limonite geodes (Museum No. G. 1308) of unusual 

 character was presented to the Museum by General G. Murray Guion. 

 According to General Guion the geodes are found in clay in the bottom 

 of a "draw" or ravine at Muscogee, Indian Territory. These spec- 

 imens are composed essentially of limonite with turgite and consist of 

 a crust, a core and a central cavity. They are of the irregular discoid 

 form with smooth exterior which characterizes a common type of 

 siderite nodule. They are of moderate size. A typical specimen 

 (Plate XXIV, Fig. 4) weighs 270 grams, has a diameter which varies 

 from 10 to 12 centimeters and a thickness of 4 centimeters. When 

 the specimen lies flat its horizontal projection is a decidedly irregular 

 oval. All vertical projections and sections are ovals, slightly irregular 

 but symmetrical with respect to the major diameter. Some specimens 

 possess th cker and some thinner forms than this. The surface is 

 smooth except for such roughness as is due to scaling of the lamellar 

 crust. The color is light gray with dark brown stains. Some 

 specimens are coated with a firmly adherent yellow ochreous clay in 

 which they appear to have been imbedded, while many specimens 

 are perfectly free from this coating. The specimen shown in the 

 illustration is enclosed in a light-colored laminated crust. Inside the 

 crust and sharply separated therefrom, is the main portion of the 

 geode, a hard, red and yellow, concentrically banded, agate-like mass 

 of limonite and turgite. The center is occupied by a small cavity 

 which varies in shape and size in different specimens, and suggests in 

 outlines the central cavity often found in agates. 



The shell is from 3 to 7 millimeters in thickness. Its external 

 color is gray to brown; fractured surfaces are ight gray with dark 

 brown and limonite yellow areas. The outer portion of the crust is 

 almost universally light gray, while the inner parts contain more 

 of the darker areas. 



The crust is strongly laminated, especially in the outer portions. 

 The individual laminae, which are somewhat under a millimeter in 



