REVIEWS. 705 
it is worthy of notice that, while the geodes are always surrounded 
by material rich in alumina, no crystallized forms of this mineral 
have yet been discovered in them. Geodes of fine quality oc- 
cur in Missouri, and Prof. White reports calcareous geodes, some 
of them entirely free from silica, in a soft magnesian lime-stone, 
in North Eastern Iowa. 
Any one who for the first time sees a fine geode and notes its 
regular form, its uncouth exterior and brilliant interior is sure to 
ask “ How was it made?” The more specimens he sees the more 
is his curiosity excited, and if he visits the locality and sees them 
in situ his wonder and interest increase to the highest pitch. 
There are very many difficulties to be overcome in trying to ac- 
count for the presence of the geodes and their formation, and it 
may be only presumption to attempt that which so many skilful 
mineralogists have passed in silence. However, a careful study of 
the geodes, both in cabinets and in their native bed, has convinced 
me that they must have been formed in some such manner as that 
here presented. What now constitutes the geode bed, was at 
one time a mass of plastic clay filled with siliceous and calcareous 
fluids. That it was plastic is shown by facts obvious to any 
one who has ever visited the locality. If now this mass were 
acted upon by steam or some other vapor, or gas, as dough is 
acted upon by carbonic acid, it would like the dough be filled 
with cavities of all sizes of a more or less spherical form, and 
irregularly distributed through the mass. Into these cavities the 
solutions of silica etc. would filter and in some cases crystallize, 
in others simply deposit the solids held in solution, according to 
circumstances. The form, arrangement, etc., of the various sub- 
stances would be determined, I suppose, by their densities and the 
laws of crystallization. After all the materials had hardened, the 
soft brittle clay would easily separate from the harder filling of the 
cavities and this would then fall out shaped by the mould in which 
- formed. 
REVIEWS. 
THe PEABODY MUSEUM OF AMERICAN ÅRCHÆOLOGY AND ETHNOL- 
- oay.*——The fourth annual Report contains much that is of gen- 
Af; af American Archæ- 
* Fourth Annual Report of tt t f the P 
ology and Ethnology. Boston, 1871. 8yo. p. 27. 
