Sorbv : The Origin of the Cleveland Ironstone. 355 



carbonate of lime, but in others they are chang-ed to carbonate 

 of iron ; the difference beings apparently due, in some instances 

 at least, to the kind of shell. The microscopical investig-ation 

 of a thin transparent section of the stone shows far more clearly 

 that the minute frag"ments of shell have been similarly altered ; 

 the replacing" carbonate of iron extending", as yellowish obtuse 

 rhombic crystals, from the outside to a variable distance inwards, 

 often leaving" the centre in its orig"inal condition, as clear colour- 

 less carbonate of lime, thoug"h in many instances the whole is 

 chang"ed. The oolitic grains likewise, have such peculiarities 

 as indicate that they were altered after deposition. 



In order to illustrate this fact of the replacement of carbonate 

 of lime by carbonate of iron, I subjoin an analysis of a shell 

 from the inferior oolite of Robin Hood's Bay, the composition 

 of which, as dried at the ordinary temperature, I found to be : — 



Carbonate of protoxide of iron ... ... ... ... 78.0 



,, lime ... ... ... ... ... ... 5.2 



,, magnesia ... ... .. ... ... 3.1 



Peroxide of iron ... ... ... ... ... ... 10.9 



Water 2.1 



Carbonaceous matter... ... ... ... ... ... 0.1 



Quartz substances ... .. ... ... .. ... 0.6 



Originally, such a shell would be composed almost entirely 

 of carbonate of lime, with only a small quantity of other mineral 

 substances, and no iron ; but, as will be seen, it is now almost 

 entirely carbonate of iron, with some hydrous peroxide, no 

 doubt due to the action of the atmosphere. The peculiarities 

 in microscopical structure, already described, prove that the 

 same change has occurred in the case of ai large proportion of 

 the constituents of the Cleveland Hill ironstone ; and, according 

 to the view I propose to explain its general constitution, all that 

 is assumed is that it did also ii"i a sirnilar manner affect the more 

 finely grained particles, which do not, and could not present 

 such facts as actually prove it to have been so, and yet would 

 be more perfectly exposed to such a process of alteration. The 

 general appearance of the stone agrees remarkably well with 

 this supposition ; for, when examined with the microscope, it is 

 seen to be extremely like many lirnestones in all such particulars 

 as are compatible with the subsequent alteration, being oolite, 

 with sniall fragments of shells and patches of finer granular 

 rnatter, as is the case in many oolitic limestones. 



1906 October i. 



