118 OEIGIN AND DEVELOPMENT OF ORE-DEPOSITS. . 



and sometimes this concentration lias been so extreme as to bring 

 the deposits into a condition resembling "bedded veins" (k). 



The transference and re-arrangement of pre-existing mineral 

 matter is still more marked in the case of the altered dolomites 

 at one time largely worked for manganese in the neighbourhood 

 of Ashburton, and described by Mr. E. J. Frecheville in 1884* 

 as examples of local concentration and re-arrangement of origin- 

 ally manganiferous beds. From the analyses presented by Mr. 

 Frecheville, it seems that the concentration has been of a chemical 

 and residual nature — carbonates of lime and magnesia have been 

 carried off in solution, while the carbonates of iron and mangan- 

 ese present have been converted into peroxides. f The deposits 

 of manganese at Oombemartin, Newton Abbott, and Veryan have 

 probably had a similar origin. 



In other mining regions such " contemporaneous ore-beds" 

 are extremely common ; reference has already been made to the 

 copper schists of Mansf eld — 1 may also refer to the copper slates 

 of Wicklow, where the "sulphur course" displays the same 

 schistosity with the "country rock," and to the cupriferous shales 

 of Hon-peh, in China.]: 



The "segregated veins" of Phillips (Ore Deposits, p. 90) 

 seem to have much affinity with the ore-beds above described, 

 but they are usually much less regular in thickness. I do not 

 know of any well-characterised example in the West of England, 

 unless the E.W. "lodes" at the Parka mines near St. Columb, 

 hereafter to be described, are such. 



Examples of impregnated stratified deposits (group e) are 

 not very numerous in our mining region, but they are not 

 altogether wanting. The evidence of cupreous impregnation in 



* Trans. Roy. Geol. Soc, Corn., x, 217. 



f These and some associated derivation berl shave been worked for many years 

 for umher, of which 2,766 tons were produced in 1883. 



J These consist of soft argillaceous rock filled with "light-green films and 

 specks of malachite and chrysocolla in the cracks of cleavage and stratification" — 

 or else siliceous bands " containing specks of cuprite with the green oxidized 

 minerals also conformable— and occasional pockets of " pure copper-ore" (impure 

 oxides with a little unchanged sulphide permeated by streaks of carbonate, and 

 assaying up to 70 per cent) . There are no mineral veins — the primary sources of 

 the mineral are sedimentary, and the patches must be ascribed to the redeposition 

 of the metal by infiltrations of solutions derived from other sources of unoxidized 

 minerals in the adjacent rocks." See Becher, Quart. Jour. Geol. Soc, 168, p. 494. 



