Reviews—Stanford’s Geography—Australian Geology. 423 
(4) Llandeilo Shales (Glenkiln facies). Tiddyn Diewm, Tremadoc. 
Country rock contains graptolites of the Didymograptus 
Murchisont and Cenograptus 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 deals with the probable petrological 
and chemical history of the iron-ores. 
Evidence is brought forward to show that the ore bodies have been 
only profitable near the present surface, and when smelted with wood 
charcoal. They are always very rich in pyrites or marcassite, which 
in certain specimens from the deep termination of an adit makes 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 sulphidic minerals usually grow out from or around 
quartz grains or other clastic fragments of country rock or of earlier 
formed pisolite grains. During oxidation the sulphidic material 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, and 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 heat 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 feet metamorphic aureole in the Llandeilo shale of Harper 
Quarry, Builth, shows that such concentration can occur on a small 
seale, while the finding of considerable lenticles of iron-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 recrystallized pyrites does tend to take on a pisolitic 
habit. 
gy dan Wh Ge dE WW Se 
AUSTRALIAN GroLoeyY. 
Sranrorp’s ComeenpiumM oF GerocrapHy, ETc. New issue, 1907. 
Avstrazasira. Vol. [: Ausrratia anp New Zeatanp. By J. W. 
Grecory, D.Sc., F.R.S., Professor of Geology in the University 
of Glasgow. (London: Edward Stanford.) 
!Y\HE whole of this volume abounds with geographical and geological 
references, as might be expected from the author, and is also 
exceedingly well illustrated with maps, etc. Chapter iv, ‘‘ The 
