40 FreLp CoLuMBIAN MusEuM—GEOLOGY, VOL. III. 
selves to any considerable extent, as in this case there would be- 
instead of individual geodes, a cellular honeycomb structure of 
limonite enclosing clay in its meshes. 
The limonite has been deposited principally, perhaps wholly, 
where the ferruginous waters have soaked into the clay as coatings 
upon the individual clay particles. Not filling the joint fractures, the 
limonite coatings of adjacent specimens do not commonly adhere. 
When they do adhere, compound or twin geodes are formed. The 
source of the iron cannot be determined, as practically nothing is 
known of the mode of occurrence of these objects. Except for the 
outer form, these objects simulate closely those concretions that are 
assumed to originate in the decomposition of a pyrite nodule and the 
deposition of the resultant oxide of iron around it. It is a question 
if many of the hollow iron concretions may not be geodes of this 
nature, although it is certain that not all are. If the deposition of 
iron oxide continued long enough, such a deposit would become one 
of argillaceous limonite. 
NODULES FROM THE CHALLENGER AND ARGUS BANKS IN 
THE ATLANTIC OCEAN 

While engaged in collecting fish for this Museum, Dr. Tarleton 
H. Bean, on the rath of October, 1905, dredged from the Challenger 
Bank sixty-four calcareous nodules. The following day he dredged 
from the Argus Bank twenty-eight similar nodules. These specimens, 
now a part of the Museum collections (Museum Nos. G. 1323-30), are 
sufficiently problematic in character to be worthy of some study, 
especially as, if of a certain character, they would have an important 
bearing upon geological and geographical problems of great interest. 
The Challenger Bank, whence the larger number of specimens 
were secured, is a shoal of from five to ten miles diameter, rising 
abruptly from the depths of the sea to within twenty-four to thirty 
fathoms from.the surface. The Bank lies thirteen miles southwest 
of Gibbs Lighthouse, Bermuda, and is separated from the Bermuda 
Bank by a space of three and one half miles of deep sea, where 
soundings exceeding 1,000 fathoms have been taken. The Argus 
Bank is a shoal of similar dimensions and depth of water about 
twelve miles southwest of the Challenger Bank, from which it is 
separated by a trough of five hundred fathoms depth. There is no 
shallow water connection between these two banks, nor with any 
other shoals or land. 
