CLINTON OOLITIC IRON ORE 771 



solution to pass through this compact calcite (of the matrix) until the 

 spherule is reached., and then begin to deposit the iron and replace the 

 calcite?" (3) If the ore resulted from the alteration of a limestone, the 

 alteration would begin at the outside of the spherules and work toward 

 the center. After the outer laj^ers were altered they must have served to 

 protect the interior and some trace of the interior should be preserved. 

 (3) If the ore was produced by substitution, it must have been precipi- 

 tated first as iron carbonate, and if so it is difficult to account for its 

 present composition. 



These arguments against the replacement of original calcareous oolites 

 may be answered categorically : 



(1) If the original calcareous oolites were formed as such oolites are 

 being formed today under all conditions, they were built of calcium car- 

 bonate in the mineral form aragonite, a mineral much less stable than 

 calcite, and hence the first to be attacked by circulating solutions of any 

 kind. These aragonite oolites would, therefore, most surely be replaced 

 before any of the surrounding matrix was replaced by the ferruginous 

 and silica-bearing solutions. Many fossils were originally composed of 

 aragonite, and others which are not so constituted have the calcite of 

 their skeletons in a more soluble form than the crystalline material which 

 fills the interspaces among them. This is true of the fragments around 

 a modern coral reef and was, undoubtedly, true in Clinton time. The 

 fossil shells were, however, less easily affected than the aragonite spher- 

 ules, and this is clearly indicated by the fact that in many of the fossil 

 fragments the central interior part has not been replaced. 



(2) The outer, earliest replaced zones do not form impOTvious cases, 

 however dense they may appear under the microscope. Circulating solu- 

 tions can penetrate these outer layers and alter the interior just as cir- 

 culating solutions can enter a spherical agate, or a geode, and build up 

 the interior after the outer shell or case has been completed. It is well 

 known that fossils may be molecularly replaced by silica or other sub- 

 stances, such as pyrite, after they have been completely incased in a 

 matrix. If the numerous fossils of the New Scotland beds of the Lower 

 Devonian of eastern New York can be completely replaced by silica while 

 they are incased in a matrix of limestone, or if the cephalopods of the 

 Middle Devonian can be replaced by pyrite, is it not reasonable to argue 

 that the silica-bearing ferruginous solutions could penetrate the granular 

 crystalline calcite matrix of the Clinton oolites and fossils to bring about 

 their replacement ? 



(3) It is clearly stated by those who have studied these ores most 

 carefully, both in New York and in the more southern outcrops, that 



