102 GEOLOGICAL NOTES ON COAL AND IRON ORE 



extent that smelting of iron on a large scale will prove 

 remunerative. 



How ores of this type can be formed in some cases 

 is illustrated to a certain degree by the conditions observed 

 within those ores which occur higher up in the coal-series, 

 and which are met with right up in the Upper Carboni- 

 ferous. They are of rather minor practical importance and 

 are seldom exploited. 



Specially characteristic for the occurence of these latter 

 ores is the fact that they are always intimately connected 

 with marine sediments. They have usually the shape of 

 round nodules or lenses of up to head-size, of argillaceous 

 sphero-siderite which shows the common concentric structure 

 or else pyritic, rusty limestone, and limonite. Continuous 

 thin layers of Iron Hydroxide containing nodules of black 

 flint, rusty limestone and concretions of sphero-siderite, have 

 also been observed. 



Similar conglomeratic ore -formations, though of smaller 

 dimensions, occur almost as a rule just above most of the 

 marine limestones which have been observed within Yang- 

 Hsii-Hsien (the Tai-Yuan district) and seem always to be 

 intimately connected with their marine substratum. 



It is not impossible that part of these iron ores have 

 in a secondary manner originated from the marine sediments 

 through a metasomatic solution of part of these latter rocks. 



Characteristic for most of the marine lime-sediments 

 is their comparatively large tenure of Iron Sulphide (Pyrite). 

 This appears microscopically as concertions of pyrite and 

 pyritized fossils enclosed in the sediments. At the weather- 

 ing of the latter a considerable amount of limonite is formed. 

 The tenure of pyrite is specially high in the horizons of 

 calcareous shale which are not infrequently found as inter- 

 calations in the marine limestone. These layers of pyritic 

 shale are not seldom characterized by a unique abundance 

 in fossils. 



Nothing in the occurence of the iron sulphide indicates 

 that it should be a secondary formation, caused by impreg- 

 nation of the sediments by iron-bearing solutions. The 

 sulphide has probably been precipitated simultaneously with 

 the deposition of the marine sediments and at a later stage 

 a molecular re-arrangement has taken place, resulting in a 

 transition to crystallized pyrite. 



It does not seem improbable that at least part of 

 the Carboniferous iron-ores have in a secondary manner 



