TRANSACTIONS OF SECTION C. 511 
3. Considerable lenticles of ore are always associated with dark-blue or black 
shales or slates, which, nearer to the igneous intrusion, have become bleached and 
spotted through the influence of that intrusion. On the side of the ore body 
nearest the intrusion the country rock is usually little disturbed, and lies evenly ; 
but on the side remote from the intrusion the country rock is crossed and recrossed 
by planes of slickenslide, and is often intensely nodular. 
The more important stratigraphical horizons which have developed pisolitic 
ore bodies in the North Welsh district are :— 
(1) Lower Lingula Flags. Bettws Garmon. 
Black shales which underlie the grey flaggy sandstones with Lingulella beds. 
(2) Upper Lingula Flags. N., flank of Aran Mawddy. 
Adjoining shales contain Peltura scarabeoides. 
(3) Upper Arenig Shales. Moelwyn Bach; Milltirgerig Arenig ; below Llyn 
y gader, Cader Idris. 
Country rock contains abundant Didymograptus bifidus. 
(4) Llandeilo Shales (Glenkiln facies). Tiddyn Diewm, Tremadoc. 
Country rock contains graptolites of the Didymograptus Murchisoni and Cano- 
graptus gracilis zones. 
The workings of Pistyll, near The Rivals, seem to belong to a horizon higher than 
any of these, and may be among the Hartfell Bala shales. 
The second portion of the paper dealt with the probable petrological and 
chemical history of the iron ores. 
Evidence was brought forward to show that the ore bodies have only been 
profitable near the present surface, and when smelted with wood charcoal. They 
are always very rich in pyrites or marcasite, which in certain specimens from the 
deep termination of an adit make up about 60 or 70 per cent. of the rock, The 
ores are always impure, and contain much crushed, streaky, or fibrous shale 
between the pisoles. Where freshest the pisoles of sulphides show only radial 
arrangement of the constituent fibres, but there may also be concentric structures 
which are masked by the opacity of the mineral. The radial fibres of the mineral 
sulphides usually grow out from or around quartz grains or other clastic fragments 
of country rock or of earlier-formed broken pisolitic grains. During oxidation 
the sulphide is attacked in stages from the outside and passes by obscure processes 
first to a colourless and soluble green pleochroic mineral, and afterwards to fibrous 
limonite and compact magnetite. It is the differential development of the various 
stages which gives the resultant pisoles of the profitable ore their well-marked 
concentric structure. 
That all the pisolitic grains contained in the iron ores of North Wales have 
been formed as radial growths of iron sulphides is not yet clear; but the method 
of their geological occurrence will well accord with the hypothesis that they may 
be the concentration products of the non-carbonaceous colouring matters driven 
off by the beat of the intrusion from the black shales considered above. 
An occurrence of perfectly fresh masses of radial pyrites at the limit of a 
12-20 foot metamorphic aureole in the Llandeilo shale of Harper’s Quarry, Builth, 
shows that such concentration does occur on a small scale, while the tinding of 
considerable leaticles of iron pisolitic ore which are wholly pyrites, and have been 
concentrated during the turning of coal into anthracite in the Emlyn Mine, 
Llandeby, near Llandeilo, would seem to show that reformed pyrites does tend 
to take on a pisolitic habit. 
4. The Trilobite Fauna of the Shineton Shales. 
By F. Raw, B:Se., F.GS. 
The author passed in review the already known trilobites from the Shineton 
Shales and announced the occurrence of several new forms, For the material he 
