50 Mr. H. J. Carter on the 



posed of " grnnular calcspar," which has very properly been 

 considered by Prof. Nicholson {op. cit. p. 5) to be a '' secondary 

 change." 



Of this mineral transformation I possess two instances ; and 

 here I would digress for a moment to remark that in one the 

 coenenchyma thus altered is richly charged with another kind 

 of, and much larger, crystal than the calcite, of a dark brown 

 colour by reflected, but amber by transmitted light, which is 

 chiefly situated on the surface of the cavities in the calcitized 

 coenenchyma, where it contrasts strongly with the light brown 

 calcite not only in size and colour, but in its " regular " 

 tetrahedral summit, which for the most part projects into the 

 empty interstices of the coenenchyma thus transformed (fig. 4, 

 dd). 



To examine these crystals more particularly a fragment of 

 each specimen of the transformed coenenchyma like fig. 4 was 

 treated with nitric acid, when the whole of the calcite in each 

 was dissolved with strong effervescence, leaving in one 

 instance nothing at all and in the other a great number of the 

 dark brown crystals mentioned, most of which presented 

 beautifully defined tetrahedral summits^ and many the corre- 

 sponding part also, thus forming " regular octahedrons " 

 about l-450th inch in diameter. These, although numerous, 

 were too small for one to subject to the reducing-flame of a 

 blowpipe (for I should have blown them away), otherwise the 

 residue would probably have been attracted by the magnet, 

 and thus, as in the case of glauconite, of which they appeared 

 to me to be but another form, as will presently appear, they 

 would have been proved to have in like manner been com- 

 posed of iron. 



I was led to this view, first by finding crystalline grains 

 of the same colour mixed with those of glauconite side by side 

 in Ih'Uorella', secondly, because in the so-called "black grains" 

 now forming part of the sea-bed between the north of Scotland 

 and the Faroe Islands, to which I have already alluded 

 (' Annals,' I. c. p. 181), the transition of the brown colour 

 of some of the casts of the Glohigerince &c. into green glau- 

 conite indicates a preliminary stage only to the latter ; thirdly, 

 by the presence among the dissolved-out crystals of the cal- 

 citized coenenchyma of the cast of a Glohigerina composed of 

 the same kind of mineral ; and fourthly, because where these 

 crystals have become disintegrated in this transformed coenen- 

 chyma they have left an iron-rust stain. My inference there- 

 fore is that they are of the same composition as that of glau- 

 conite, oidy of a different colour. Further than this I cannot 

 go, and therefore must leave the question for mineralogists to 



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