SOME MET A SOMATIC CHANGES IN LIMESTONES. 59 



carbonate of lime. Smyth (12) has shown, however, that 

 true oolitic iron-ores do occur in the Clinton group, for 

 example, at the type-locality. Each grain there consists 

 mainly of concentric coats of iron-oxide with a central 

 nucleus of a rounded quartz-granule ; but, on removing 

 the iron-oxide with acid, a transparent siliceous skeleton 

 is left, showing a concentric structure, often with ten or 

 more distinct layers. From this intimate association of 

 silica with the iron-oxide, and from other considerations, 

 Smyth concludes that this and some other iron-ores de- 

 scribed were formed directly as such, and not by meta- 

 somatism from limestones. Some of the difficulties in 

 the way of this view are discussed in Kimball's paper (9), 

 while Smyth has given in another place his reasons for 

 maintaining his view of the probable origin of these 

 Clinton iron-ores (13). It may be observed that the 

 presence of a siliceous skeleton in oolitic grains now com- 

 posed of ferric oxide or hydrate is a feature of wide-spread 

 occurrence, and is found in ores undoubtedly representing 

 limestone strata. Further, it is met with in oolitic lime- 

 stones which have not undergone any ferruginous replace- 

 ment, and is, in this case, possibly only a stage in the 

 complete silicification of the rock. 



The oolitic iron-ores of Jurassic age in Lorraine, as 

 described by Bleicher (14), have characters which may 

 probably be taken as typical of many other occurrences. 

 As regards their origin the author expresses no opinion, 

 but contents himself with quoting Braconnier's supposition 

 that the material (now haematite) was deposited as chalybite 

 brought in solution by water containing excess of carbonic 

 acid. Each grain seems to consist of concentric coats of 

 ferric oxide, more or less argillaceous, investing a central 

 grain. The latter is often organic, consisting of a fragment 

 of a bryozoan, foraminifer, or shell, more or less mineralised 

 and impregnated with iron. The grains, when treated with 

 acid to remove the haematite, and heated, became white or 

 grey and transparent. They then consisted merely of a 

 nucleus of siliceous material surrounded by numerous delic- 

 ate concentric shells, some thin and clear, others cloudy 



