356 PROCEEDI^'GS OF THE GEOLOGICAL SOCIETY. [April 22, 



they generally both assume a brown colonr, and are not so easily 

 distingiiislied. 



Of the colour of the carbonate of protoxide (fig. 3, Plate XI.) the 

 cold grey of Purbeck marble and some limestones may be mentioned 

 as examples. Between this cold-gTey and the yellow and brown 

 varieties every gradation of colour occurs, in proportion to the ex- 

 tent of admixture with the hydi'ous sesquioxide, which is readily 

 formed as a secondary product by the peroxidation of the carbonate. 



The hydi'ous sesquioxide, which occurs so universally as the 

 colouring-matter of yellow sandstones, appears often to have been a 

 secondary product, formed by the hydration of the anhydrous sesqui- 

 oxide ; and the carbonate of protoxide of iron taken up in a state of 

 solution by an access of carbonic acid^ becomes immediately on rede- 

 position converted into the hydrous sesquioxide, a familiar example 

 of which occurs in the formation of ochreous tufa on the deposition 

 of the carbonates of lime and iron from carbonated springs. 



In contrast with the unstable character of the carbonate of iron 

 deposited from a carbonated solution, may be noticed the permanent 

 condition of the mineral carbonate occurring in the form of segre- 

 gated nodules in, and evenly distributed through, strata charged 

 with carbonaceous matter — a fact to be referred to in considering the 

 probability of the bleached patches in red beds being due to the 

 presence of organic matter. 



Of the strata containing protocarbonate of iron may be enume- 

 rated the grey beds of the Coal-measures, the Oolites, and the Ter- 

 tiaries, of which the following are some analyses. 



Analysis I^o. 30, of Grey Clay from Bovey Tracey Lignite deposit. 



Protoxide of iron 0'49 per cent. 



Analysis No. 31, of Ganie Clay(Pire-clay), Shropshire Coal-measures. 

 Protoxide of iron 1 •48 per cent. 



Analysis '^o. 32, of Pennystone Clod, Shropshire Coal-measures, 



BenthalL, near Broseley. 



Protoxide of iron 3-75 per cent. 



These determinations do not include the carbonate of iron occur- 

 ring as segregated nodules. 



Analysis No. 36, by Dr. Yoelcker, of Grey Clay, Coal-measures, Wyre 



Forest, Worcestershire. 



Iron, mostly as protoxide, but with, traces of basic 



sulphate of sesquioxide and sulphate of protoxide 3"88 per cent. 



Bisulphide of ii'on... {gSphlir!" olos) ^'''^^- " 



Analysis No. 37, by Dr. Yoelcker, of London Clay, Bawdsey Cliff, 



Suffolk. 

 Iron, mostly as protoxide, but with traces of pro- 



tosulphate and basic sulphate 1 -68 per cent. 



Bisulphide of iron ... { g^^^pi^;;;;;; ^'Ml] ^'^^^ - 



