38 Field Columbian Museum — Geology, Vol. III. 



thickness, are very brittle and break readily in some instances into 

 little, straight-sided rhombs which are not uniform in shape or size. 

 The hardness of this crust is about that of calcite. In appearance 

 the material of this crust resembles a siderite partially altered to 

 limonite. A chemical test, however, proves it to be limonite mixed 

 with clay and a very little calcite. 



Inside this shell is the core, which comprises the principal mass of 

 the specimen. This core readily separates from the shell when the 

 geode is broken. It consists of hard red turgite, banded concen- 

 trically with limonite. (Plate XXIV, Fig. 3.) The red portion 

 forms by far the larger part of the core. The hardness of the core 

 like that of the crust is about that of calcite. This core is of a smooth, 

 earthy texture. It rubs off sufficiently to soil paper readily. The 

 agate-like banding is disposed somewhat symmetrically with reference 

 to the centre and the outside. A section of the core presents an 

 annular form. The centre of this ring is occupied by a broad red 

 band, outside and inside of which are thin, alternating bands of 

 yellow and red, while the broad central red band is itself made up of 

 a multitude of minute, almost invisible bands of two shades of red. 



The central cavity is small in proportion to the size of the geode. 

 One specimen which has been sawn through the centre presents a 

 section of an average diameter of 6 centimeters. In this specimen 

 the section of the cavity occupies a space of 15 by 5 millimeters. 

 •The section of the opening has the form of an irregular pentagon 

 with sharp angles suggesting a crystal outline which is common 

 among agates. The cavity in this instance has a dark brown, slightly 

 iridescent coating of botryoidal limonite with two small areas of 

 colorless, transparent opal also botryoidal A thinner specimen 

 of about 5 by 25 millimeters section when sawn through the centre 

 reveals the central cavity reduced to a mere slit of 2 by 10 

 millimeters. This cavity is in the red turgite and has no limonite 

 coating. It has, however, a partial coating of an opaque white 

 powder, the nature of which has not been determined. 



Composed of quartz, these specimens would be typical agates. 

 Therefore it is most probable that they were formed in the same way 

 as agates by the deposition of oxides of iron instead of silica. As in 

 the case of agates slight changes in the conditions of deposition cause 

 changes in the color and porosity of silica deposited, so in this instance 

 slight changes in the surroundings or in the mother liquor have 

 caused alternate depositions of more and less hydra ted oxides of iron. 

 Further discussion of the origin and nature of these objects would 



