3G6 PROCEEDINGS OF THE ACADEMY OF 



The ore deposit is perfectly conformable with the strike and dip 

 of the coal bed ; mining operations have developed the presence 

 of coal above the ore, and, no doubt, would reveal it below the 

 level of the drift, if a shaft were sunk to the requisite depth. The 

 presence of geodes of fine quartz crystals and chalcedony, as also 

 the presence of pyrite, excludes all idea of igneous origin (a pri- 

 ori), and makes the ore deposit analogous to the filling of fissure 

 veins \>y the process of infiltration. The infiltration ma}^ have 

 been synchronic with the deposition of the organic matter forming 

 the coal bed, as undoubtedly the masses of iron carbonate and 

 iron oxides in the coal beds of many localities were ; or it may 

 have taken place b}^ replacement at a later period. To decide 

 about these two ways, it would be necessary to examine the de- 

 posit more closely than has apparently been done. But either of 

 them is consistent with the chemico-geological theories now held 

 by geologists. 



Philadelphia, August 14th, 1875. 



Mr. Gabb remarked that while the observation given in Mr. 

 Berthoud's paper was interesting, the writer's deduction was evi- 

 dently at fault. From the description of the facts as given, it 

 seemed to be a case of replacement by segregation, similar to 

 that which occurs in the manganese mine of "Red Rock" in the 

 bay of San Francisco, where beds of metamorphosed cretaceous 

 shale are pseudomorphed by segregated masses of black oxide of 

 manganese; in many parts the original bedding of the siliceous 

 rock being retained, but the material consisting of nearly pure 

 metallic oxide. 



