360 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [March 13, 



fissures of the containing rock, and its included rocky fragments, render 

 it probable that it is a true injection-vein or dyke. I could not observe 

 that any unusual degree of metamorphism of the containing rock oc- 

 curred in the vicinity of the ankerite. The walls of the vein, however, 

 and its included fragments often consist of a peculiar olive slate with 

 shining surfaces, and approaching in appearance to a steatite schist. 

 This slate contains silicate of magnesia, though not in large propor- 

 tion; and possibly its composition and appearance may be due to 

 the eruption of a highly magnesian rock such as the ankerite. As I 

 did not, however, give very particular attention to this circumstance 

 when on the ground, 1 am not prepared to affirm that this magnesian 

 slate is confined to the vicinity of the vein. 



True spathose iron is of rare occurrence in the outcrop of the vein. 

 At the Acadia Mine, where it occurs in intimate mixture with the 

 ankerite, it contains about twenty per cent, of carbonate of magnesia, 

 approaching in this respect to the species Mesitine spar, though its 

 proportion of magnesia is much smaller. It also contains about six 

 per cent, of carbonate of lime. This spathose iron is evidently of con- 

 temporaneous origin with the ankerite. 



The yellow ochre which aboimds in the superficial parts of the 

 deposit, and which at Cook's Brook extends to a considerable depth, 

 is evidently a product of the decomposition of the ankerite and 

 spathose iron, under the influence of air and moisture. The latter 

 substance being the more easily decomposed of the two, and being 

 found in some places mixed with the ankerite, it is possible that a 

 larger proportion of the yellow ochre has resulted from its decompo- 

 sition, than would be inferred from the relative proportions of ankerite 

 and spathose iron at the surface. No evidence appears that any con- 

 siderable portion of the yellow ochre can have resulted from the de- 

 composition of metallic sulphurets. 



The veinlets, nests, and disseminated crystals of specular and 

 magnetic iron ore, which are scattered through the vein, are probably 

 results of segregation and sublimation ; and were probably intro- 

 duced at the same time with the ankerite. 



The red ocTirey iron ore, which abounds in most parts of the vein, 

 is, in reference to its origin, one of the most difficult features of the 

 deposit. In some parts of the vein it is a very pure peroxide of iron. 

 A specimen analysed by me gave 98 per cent, of that substance. 

 By gradual additions of the carbonates of iron, lime, and magnesia, 

 it passes into spathose iron or red ankerite. Its earthy and porous 

 texture, its traces of crystalline form, and its connection with the 

 minerals just named, point to the only theory of its origin which I 

 have been able to form — that of metamorphism of carbonate of iron 

 by heat. This substance is in appearance a roasted carbonate of iron ; 

 it may be seen to pass by insensible degrees into that mineral, and 

 it often retains traces of its external form and cleavage. 



If we suppose that the vein originally contained a considerable 

 quantity of the carbonate of iron, and that after it was filled and its 

 contents consolidated, heat accompanied by access of oxygen was 

 applied, we have the conditions required for the production of 



