40 Field 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 pyr'.te 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 12th 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. 



