SCIENCE-GOSSIP. 



93 



two outcrops of the same bed are crossed, and in 

 one there, at the head of the valley near " the 

 Pass of the Men of Ardudwy," is a large mine, 

 with tramways and inclined planes laid down, but 

 at present unworked ; here the seam has been 

 mined in places some way into the hill. In the 

 valley of the Artro, itself about two miles 

 from Llanbedr, the strata are horizontal, and 

 the bed just below the surface has been eaten 

 into with a network of tunnels and connecting 

 galleries. 



In many places the bed seems extremely impure, 

 and beyond removing the surface-drift no attempt 

 has been made to mine it. The deposit varies 

 from a manganese-coloured slate to an almost 

 white distinctly stratified ore, consisting of carbo- 

 nate of manganese, with some twenty per cent, of 

 siliceous matter. In one of the workings a bed of 

 quartz grit occurs with a cement of oxide of 

 manganese. Four feet is about the maximum 

 thickness of the seam, and two and a half the 

 average. It is well defined from the beds above 

 and below ; in most cases the change from an 

 ordinary slate to the ore is quite abrupt. 



On examining the ore as it lies about the w-ork- 

 ings (for no quarrying was in progress during my 

 visits in 1894 and 1895) o°e sees two varieties — a 

 grey rock, often very siliceous, consisting mainly 

 of the carbonate, and a soft pliable black form, the 

 hydrated oxide. The faces of the headings are all 

 stained a dull \-iolet, and the same tint is characte- 

 ristic of the broken masses at the quarry mouth. 

 The black variety is only found along the outcrop 

 where the bed has been penetrated by fissures and 

 cracks, along which it eats its way into the grey 

 ore, forming a box-like covering to kernels of 

 unaltered ore, the line of demarcation being per- 

 fectly sharp. 



I was favoured by Mr. J. Abraham, of the 

 Barmouth mine, with analyses of the two varieties 

 by Messrs. Patterson and Stead, the leading points 

 of which I produce. 



Dried at 212 degrees. 

 Black. White. 

 Peroxide of manganese - - 3448 — 



Protoxide of manganese - - 11 83 29-48 

 Peroxide of iron - - - 4-07 2-21 



Protoxide of iron - - - — 103 



Alumina - - - . . 306 2 39 



Lime- - - - . . 207 4-53 



Sihceous matter (see below^) - 3800 3495 

 Magnesia - - . . . 0-31 0-64 



Carbonic acid - - - -1-40 23-00 

 Combined water - - . 4-60 1-35 



Sulphur ----- 08 08 



Phosphoric acid - - - 012 010 



The siliceous matter when further examined gave : 



Dried at 212 degrees. 



Silica 



Alumina - 



Protoxide of manganese 

 Peroxide of iron 

 Lime - - - - 

 Magnesia - 



Black. White. 



2225 19-40 



825 585 



5-21 744 



145 145 



0-62 056 



021 021 



IOO-02 99"76 

 This gives metallic manganese - 35 7^ 28-60% 



These analyses sho-.v -^vhat seems evident from 

 the examination in the field, that the black ore 

 is a product of alteration of the grey ore due 

 to weathering. The grey ore contains in its 

 ordinary state a certain amount of silicate as well 

 as carbonate of manganese ; it is, as Mr. Goodchild 

 says, an impure mixture of dialogite and rhodonite. 

 The alteration along the outcrop seems to me to 

 exactly resemble the change which has occurred in 

 the ironstone beds of the Northampton Sands. If 

 one reads the account of these changes, in the 

 " Geol. Survey Memoir," for example (" The 

 Jurassic Rocks of Britain," vol. iv., p. 494), the 

 explanation given there would exactly describe the 

 changes in these Welsh ores, reading the word 

 manganese for iron. The figure given of the brown 

 ore, enclosing unaltered portions of carbonate, com- 

 pletely represents -^vhat is usually to be seen in 

 these manganese mines. 



I might add that the change seems to result in a 

 diminution of bulk, so that more opening is given 

 for the infiltration of surface waters. 



It would be interesting to know whence came 

 these large deposits of manganese. The black 

 oxide is widely disseminated through all the 

 stratified rocks, and I recently found a lenticular 

 mass, some inches thick, in a Pleistocene flint 

 gravel, resting on the chalk ; deep-sea soundings 

 also prove its presence in the ocean depths. The 

 Cambrian beds, in which the ore is found, are them- 

 selves derived from the denudation of previously 

 existing beds, about which we know so little. 



The mines seem to be only worked at inter\'als, 

 as the price of the ore permits. If 25s. per ton 

 "placed on the rails," allows a profit to some 

 favourably-situated mine, there must be others in 

 the remote glens of Ardudwy, when this would be 

 balanced by the cost of transit for several miles 

 along mere cart tracks. 



The iron-works of North Wales, when it is used 

 in the manufacture of ferro-manganese, seem to be 

 the usual destination of the ore. 



Harpendcn, Herts ; July. 1896. 



Science at Nottingham. — We have received 

 the third supplement to No. 3 Class List (Science) 

 of the Borough of Nottingham Free Public 

 Reference Library. It is made up to April last. 

 There are many additions since the last catalogue 

 was issued. 



E 4 



