38 Fretp CoLtuMBIAN Mus—EuM—GEo_ocy, 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 hmonite 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 symmetricallywith 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 1s 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 hydrated oxides of iron. 
Further discussion of the origin and nature of these objects would 
