1882.] 145 [Julien. 



fibres of actinolite. Still there are localities where the upper sur- 

 face of a dunyte outcrop in North Carolina is clothed with forest. 

 Here the aluminous content of the soil, shown by its analysis, 

 indicates an increment of foreign material in some way, perhaps 

 by sand or dust blown by the wind. 



Alteration. 



The various processes of alteration, which have attacked and 

 modified the lithological character of dunyte, are as important as 

 they are novel and interesting. Although substantially the same 

 reactions and results are involved, the discussion of these pro- 

 cesses may be separately considered, with reference to that which 

 took place in fissures, and to those which progressed throughout 

 the rock, within its interstices. The respective results were, from 

 the first process, the formation of veins — from the others, the 

 conversion of the dunyte, partially or completely, into different 

 rocks. 



Veins. These vary in form, from vertical sheets, intersecting 

 the rock to unknown depths, to elliptical or lenticular pockets 

 with vertical axes ; in thickness, from mere films to a width of 

 two meters. In the veins of ordinary type, which are very com- 

 mon, the walls are lined with successive laminae of actinolite in 

 transversely fibrous crusts, sometimes partially or wholly altered 

 into talc, and the interior is occupied by ripidolite. The latter 

 mineral almost invariably serves as the matrix of the corundum, 

 with its associated minerals, these corundum veins being particu- 

 larly abundant and large in the southern part of the dunyte-belt. 

 In regard to the varied series of minerals which are found in asso- 

 ciation with the corundum, and in regard to their paragenetic 

 relations, it will suffice for the present purpose to refer to Dr. 

 Genth's valuable paper, the general accuracy of which on these points 

 my field-observations everywhere confirm. It may be added that 

 brownish-green enstatite is a common member of the vein-series ; 

 sometimes occurring in huge masses, nearly a meter in diameter, 

 lined with actinolite ; sometimes in minute disseminated granules ; 

 and less commonly in brilliant bronze-colored scales, to which the 

 name bronzite is more pertinent. 



Indigenous Alteration. Four common modes of alteration 

 may be observed throughout the dunyte-belt, in all stages of each 

 process. 



PROCEEDINGS B. S. N. H. VOL. XXII. 10 AUGUST, 1888. 



