392 DALE— CAMBRIAN MANGANESE DEPOSITS OF [April 25, 



The nodules of 219 A 4a are discoidal in shape and vary in diam- 

 eter from ^ inch to i^ inch, with an average of about i inch and a 

 thickness ranging from ^ inch to Yi inch. The longer diameters 

 of the nodules lie in the plane of the bed. Where the nodules are 

 very numerous or crowded they are found intergrown with or over- 

 lapping each other. Specimens ground and polished often show a 

 lemniscate formed by two nodules (Figs. 14 and 15). In color they 

 are for the most part green, but may have greenish, white, or pink 

 central cores. Cross sections of the nodules reveal a distinct zonal 

 arrangement with spherical central cores surrounded by concentric 



Fig. 16. Microphotograph of manganese carbonate nodule from 219 A4a ; 

 slide 288; enlarged 6 diam. a, carbonate of manganese, lime and magnesia,; 

 b, barite ; c, barite replaced by chlorite ; d, shale. 



shells conforming to the shape of the nodule. The grain of the 

 nodules is usually exceedingly fine, impalpable or crystalline. The 

 pinkish cores are usually crystalHne and respond to the HCl test 

 quite readily, indicating some carbonate mineral. By analysis the 

 green nodules are found to consist essentially of rhodochrosite (see 

 Anal. B, page 395), while the pinkish crystalline mineral occupying 



