OEIGIJSr AND DEVELOPMENT OF ORE-DEPOSITS. 115 



decomposed — the slates here forming a kind of clay, and the 

 sandstones being largely disintegrated into sand. In these soft 

 beds are to be seen irregular layers of nodular limonite, while 

 certain less decomposed beds still contain much magnetite and 

 hornblende in an almost unaltered condition. Here, and also 

 at several points along the outcrop — which may be traced for 

 about a mile to the eastward — quantities of ochre and umber are 

 met with irregularly disposed in the decomposed mass, and 

 evidently themselves decomposition products.* 



As to the origin of these beds, Dr. Foster, in the paper already 

 referred to, remarks that " beds of iron-ore deposited contem- 

 poraneously with shales and sandstones seem to have been sub- 

 jected to a metamorphic action — probably due to the proximity 

 of the granite. The iron-ore — perhaps originally in the form of 

 beds like the Cleveland ore — has been altered into magnetite, 

 whilst the change undergone by the shales and sandstones con- 

 sists in an extreme silicification."f I quite believe that this is 

 the true explanation after a careful microscopic and partial 

 chemical examination of the rocks and associated minerals ; and 

 I see no ground for the second supposition put forward by Dr. 

 Foster (though hesitatingly) "that the apparently stratified 

 magnetite may have been formed by ferruginous emanations 

 which accompanied or followed the granitic intrusion, and spread 

 out between the planes of bedding. "| There is no evidence 

 whatever of the existence of an actual ''fissure" or "junction" 

 vein which has served for the channel for ' ' ferruginous solutions 

 or emanations." I have little doubt that the original ferruginous 

 beds consisted of carbonate of iron, that the heat from the 

 proximity of the granite, aided by water circulating through the 

 beds, has converted it into magnetite, and has also produced 

 and developed the hornblende, axinite, garnet, &c. The general 

 silicification of the fine-grained shales and sandstones seems to 

 me to have been a subsequent process. It has produced in some 

 places a kind of quartzite and in others a fine-grained banded 



* In these beds large masses of gramenite were risible at the time of my visit 

 in 1871. See Min. Mag., vol. 1, p. 67. 



t Q. J. G. S., 1875, 629. 



X p. 630. 



