394 



DALE— CAMBRIAN MANGANESE DEPOSITS OF [April 25, 



The exterior zones merge imperceptibly into the shale, a fact which 

 has some srenetic significance. 





d 



Fig. 18. Microphotograph of two manganese carbonate nodules from 219 

 A6c; slide 237; enlarged 8 diam. 



An incipient oolitic structure with spherules of hematite is com- 

 mon to the outer zones of the nodule and shaly matrix. The spher- 

 ules do not as a rule show any well-developed zonal structure nor are 

 they of very regular form. They vary in diameter from 6 microns 

 to yj microns and have an average diameter of about 24 microns. 

 Not infrequently the spherules consist of both carbonate and hema- 

 tite, the former preserving a radiating structure and abounding in 

 the more calcareous portions of the specimen, while the hematitic 

 spherules are more common in the shaly parts. 



Among the determinable minerals are calcite, which occurs as 

 anhedral grains of variable dimensions in small crystalline areas, in 

 veins, or as replacement material after organic remains such as sponge 

 spicules, etc. Carbonate material for the most part specifically inde- 

 terminable makes up the greater part of the slide. Barite is found 

 occupying the more central portions of the nodule in some sections. 

 Quartz as irregular grains occurs only in sparing amounts. Pyrite 

 is present as large and small irregular grains and masses. 



