THE ORIGIN OF BOMBSHELL ORE. 



By H. M. chance. 

 (Read May 15. 1908.) 



The term " bombshell " ore is applied by miners and iron-masters 

 to hollow masses of limonite — brown hematite — which sometimes are 

 round or oval but more commonly are of any irregular shape. The 

 " bombs " may contain water, clay, sand, quartz, flint, pyrite, siderite, 

 sandstone or decomposed slate, or may be entirely empty. Geolo- 

 gists usually speak of such ore as nodular or concretionary. A 

 careful examination of the literature of ore deposits and especially 

 of that relating to the genesis of limonite ores fails to disclose a 

 satisfactory explanation of its origin of mode of formation. By 

 many it is assumed to be similar in origin to silicious geodes, which 

 are supposed to be formed by the deposition of silica or silicates 

 upon the walls of cavities, while others describe it as of " concre- 

 tionary " origin without attempting to explain the process of forma- 

 tion or the manner in wdiich it has occluded the variety of materials 

 which are found in the interior of different specimens from the same 

 locality. That it is not of concretionary origin is evident upon even 

 cursory consideration, for concretions are masses of material ar- 

 ranged in concentric la}ers around a central nucleus. The latter 

 may be a grain of sand, a pebble, fossil, or any substance around 

 which (as a core) the concretion forms, growing from the center 

 by the successive addition of concentric rings. Concretions are 

 perhaps merely symmetrical segregations. 



The peculiarities of this ore are well described by Professor T. 

 C. Hopkins, Bull. Gcol. Soc. Am., 1890, Vol. 11, p. 477, etc., as 

 follows : 



" Nodular ore consists of irregularly rounded masses, varying in size 

 from a fraction of a pound to several hundred pounds in weight. The 

 masses are frequently hollow, but some enclose a rounded or sub-angular 

 rock fragment, which is sometimes sandstone, . . . sometimes chert, some- 

 times slate, and sometimes clay. Some shells are filled with clay and sand, 



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