272 On the Distribution of Manganese. 



This colouring matter, which is wholly superficial, and of different 

 degrees of lustre, is due to an incrustation of the black oxide of 

 manganese, and occurs independently on almost every variety of 

 stone. 



In Jameson's Edinburgh New Philosophical Journal, for July 

 1851, Dr John Davy calls attention to somewhat similar incrusta- 

 tions in England, of which he says as follows : " Though always 

 superficial, in one spot the incrustation is so thick as to be available 

 for use ; and in this instance the black oxide of manganese acts as a 

 cement, forming a bed of conglomerate, several feet thick. Whence 

 this incrustation is derived^ or how produced, is not obvious. Re- 

 stricting the view to the spots where it occurs, it might be supposed 

 to be a deposit from running water. But when it is seen that the 

 colouring matter is not to be detected on rocks in situ, the fixed 

 rocks in the course of the stream, the idea ceases to be tenable, and 

 the inference seems to be unavoidable, that the sand, pebbles, and 

 stones, so coloured, have been incrusted with the oxide before they 

 had been carried down to the spot where they are found loose, or 

 when in the form of conglomerate, that the cementing oxide has been 

 brought by water exuding from some rock or stratum containing 

 manganese in a minor degree of oxidation, and acquiring the higher 

 degree by the absorption of oxygen, and at the same time the ce- 

 menting quality." Dr Davy also infers that manganese exists in the 

 vicinity of these incrustations in large quantities, and advises special 

 inquiry in search of it. 



Before the publication of the article referred to by Dr Davy, the 

 subject of these incrustations had attracted the attention of Dr A. 

 Hayes of Boston and myself, and we believe the following to be a 

 full and satisfactory account of the origin of this phenomenon : — 



The manganese exists in almost all the igneous and metamorphic 

 rocks of New England, and I may say in other parts of the world, 

 generally as a double carbonate of lime and manganese. When the 

 waters of the springs, brooks, and rivers, flowing over these rocks, 

 become changed with soluble organic matter, in the state of crenic, 

 apocrenic, or humic acids, drained into them in consequence of rains 

 or inundations, from swamps and peat meadows, the carbonates of 

 lime and manganese enter into solution. At such times manganese 

 may generally be detected in these waters, as has been done by Drs 

 C. T. Jackson, A. A. Hayes, and others. When the water holding 

 the manganese in solution becomes broken and thrown up in the pas- 

 sage of falls or rapids, consequently exposing it to the influence of the 

 atmosphere, the manganese passes from a low state of oxidation to 

 the insoluble peroxide, and is deposited for a considerable extent upon 

 the rocks and pebbles below. It will thus be found upon examina- 

 tion, that at intervals in the bed of the stream, the stones are com- 

 pletely blackened or discoloured, while in the other places no such 

 depositions exist. Beautiful examples of this phenomenon may be 



