June, 1906. New Forms of Concretions — Nichols. 39 



appear unprofitable until their occurrence has been investigated in 

 the field. 



LIMONITE GEODES FROM THE OHIO RIVER 



A series of four hollow limonite objects (Museum No. G. 1307) 

 of rhombohedral form which were presented to the Museum by Dr. 

 W. S. Gilmore prove to be limonite geodes. (Plate XXIV, Figs 1 

 and 2.) They are described as occurring in large numbers in clay 

 upon the banks of the Ohio River about 30 miles from Owensboro, Ky. 



They are small, weighing from 28 to 64 grams. They are all of 

 approximately the same thickness, 25 millimeters, the same width, 

 25 millimeters and vary in length from 26 to 60 millimeters. With 

 the exception of one imperfect specimen they are bounded by plane 

 faces and are in form typical joint rhombohedrons formed between 

 bedding planes and three systems of parallel and intersecting joints 

 perpendicular to the bedding. Two systems of the joints are practi- 

 cally perpendicular to each other. The third system intersects the 

 others at angles varying from 40 to 6o°. In all the specimens two 

 parallel surfaces which differ in color from, and are more earthy in 

 texture and rougher than the others, are identified as bedding planes. 



The surface of the geodes is yellow on the bedding planes 

 and dull red to brown on the joint faces. Fractured surfaces are 

 dull brown and smooth, with a yellow streak at the inside edge. The 

 specimens are hollow, with thicker walls along the bedding planes 

 than along the joint surfaces. 



In one specimen (Plate XXIV, Fig. 2) the walls of the geode in 

 contact with the bedding planes have a thickness of 5 to 7 millimeters, 

 while the walls in contact with joint planes have a thickness of only 1 to 

 2 millimeters. This specimen happens to be double, the half-specimen 

 or individual to which the above measurements refer having a breadth 

 and thickness respectively of 24 and 16 millimeters The interior 

 hollows of the unbroken geodes are filled with a tough,. yellow, och- 

 reous clay, reticulated on the surface with drying cracks. 



It is very ev dent from the form and structure of these objects 

 that they are formed at the intersection of joints and bedding planes. 

 They do not represent actual open spaces, but rather are blocks of 

 clay enclosed by these fractures and modified by the introduction of 

 limonite from the exterior by ferruginous waters. These waters do 

 not appear to have deposited their iron in the joint openings them- 



