180 OEIGIN AUD DEVELOPMENT OF ORE-DEPOSITS. 



New Eosewarne, on a continuation of tlie Relistian lode — 

 considers them — though with some hesitation — to be true pebbles, 

 fallen in from above.* 



Many concretions occurred formerly in the lode at Wheal 

 Trelawney. One, consisting of a kernel of galena succeeded by 

 four layers of quartz, the fourth containing fragments of the 

 third, and coated over all with galena, is described and figured 

 by Mr Henwood.f The concretions remind one of the well- 

 known " ring-erz " of the Hartz. 



The concretionary form of tin-oxide, known as wood-tin, has 

 often a concentric, and frequently also a radiate structure, as at 

 the Garth Mine near Penzance, Wheal Metal in Breage, Penhalls 

 in St. Agnes, Prideaux Wood near Par, &c.J The former is 

 probably an original concretionary colloid structure, the latter 

 more probably a superinduced crystallization. Many of the 

 distinctly crystalline tin-stones have also a somewhat radiate 

 structure, § as in the case of the toad's eye tin from Penhalls, this 

 therefore is rather to be described as an example of the sporadic 

 crystallization to be adverted to presently. 



In all cases of concretions in sedimentary rocks it seems 

 likely that the determination of the position of any particular 

 concretionary aggregation depends upon the presence of some 

 fragment of organic substance which has served as a nucleus. 

 We can hardly suppose a similar determining cause in the case 

 of concretions in eruptive rocks or in veinstones, although 

 Mr. Moore long ago proved the existence of numerous organisms 

 in the flucans of the lead veins in the Mendips. || 



Spheroidal structure is really a form of jointing, but it often 

 occurs with concretionary structure, and is often confounded with 

 it. This structure is most common in eruptive rocks, although 

 it also occurs in sedimentary rocks. 



* Eeport M.A., 1866. 



t Trans. E.G.S.C, viii, p. 703. 



X See Plates viii, ix, x, xti, Cornish Tinstones and Tin Capels. 



§ Ibid, Plate v, figs. 1 and 3. 



II C. Moore " On the organic contents of mineral veins." — Brit. Assoc. Rep., 

 1869. A list of nodular concretionary bodies surrounding organic bodies 

 is given by Mr. M. H. Johnson. Jour. Quekett Microscop. Soc, May, 1875. 



