452 PEOCEEDINGS OF THE AMERICAN ACADEMY 



The last column gives the percentage corresponding to the symbol 

 Fe^ Si H^; and it is obvious that the composition of the mineral is 

 remarkably definite, and the agreement of the analyses with theory 

 very close. 



Melanosiderite is closely related to the sesqui-hydrates of iron. If 

 the silica could be regarded as an impurity, the ratio between the 

 sesquioxide and the water would be precisely that of Liraonite ; and, on 

 the other hand, if we regard the basic radical as neutralized both by the 

 silicon and the hydrogen, then the ratio would be that of Gothite. 

 Melanosiderite differs, however, most markedly from both of these 

 minerals in its specific gravity. 



Sp tir. 



Gothite 4. — 4.4 Dana's Mineralogy. 



Limomte 3.G — 4. ,, „ 



Melanosiderite 3.391 As above. 



INIoreover its vitreous lustre, its fusibility, its definite composition, 

 and, above all, its reaction with acids (gelatinizing), indicate tliat it is 

 a true silicate. The mineral which it most closely resembles, both in 

 external appearance and specific characters, is Hisingerite, but this 

 contains thirty-six per cent of silica. Melanosiderite, however, is evi- 

 dently a highly basic compound of the same class ; and the new species 

 has a special interest, arising from the circumstance that it is a definite 

 natural example of a class of compounds which are so characteristic of 

 the ferric radical. Its amorphous colloidal structure is wholly in 

 accordance with this view of its constitution. The name Melanoside- 

 rite is derived from Greek [is)M>i and ob8ijQog, and refers to the very 

 striking black color of the mineral. 



The analytical work in this investigation has been done by Mr. 

 "W. H. Melville, of the Senior Ckiss in Harvard College. 



