72 Notices of Memoirs — Irish Deer in Isle of Man. 



old waste heaps of slags and slimes. From these heaps upwards of 

 9,000 tons of lead was extracted during- the ten years ending 1880. 

 The material now being worked at Priddy has the appearance of 

 a brown earth : it contains fragments of charcoal and limestone, and 

 about 6 per cent, of lead as carbonate. Embedded here and there 

 in this material are blocks consisting of devitrified slag, partially 

 fused galena, and fragments of charcoal ; and in the cavities 

 numerous small crystals of cerussite (Pb C Og) and anglesite (Pb S 0^) 

 and, less frequently, of leadhillite. 



Leadhillite has not been before observed under such conditions. 

 In the Roman lead slags at Laurion in Greece, which have been in 

 contact with sea-water, Lacroix has noted the following secondary 

 minerals : matlockite, penfieldite, laurionite, fiedlerite, phosgenite, 

 cerussite, hydrocerussite, and anglesite. 



The colourless crystals of Mendip leadhillite have, perpendicular 

 to the perfect basal cleavage, an acute negative bisectrix with an 

 optic axial angle in air of 2B=72f°; at a temperature of 97° C, 

 2E = 70^°. The frequent twinning and the goniometric measure- 

 ments (which are, however, not very good) are not inconsistent with 

 the orthorhombic symmetry insisted upon by Miller. The basal 

 planes of complicated twin crystals always give a single sharp 

 reflected image, which is not the case with twin crystals of ordinary 

 monosymmetric leadhillite. A few crj'stals are optically uniaxial. 



There therefore seem to be three kinds of leadhillite, all of which 

 are identical in outward appearance : (a) Monosymmetric, with the 

 optic axial angle 2E=:20° ; [h) rhombohedral (?) and optically 

 uniaxial (susannite) : (c) orthorhombic, with 2E = 72;|°. 



Before 1874 the formula for leadhillite was given as PbS04.3PbC03, 

 and that now usually accepted is PbS04.2PbC03.Pb(^OH)2; but no two 

 of the several analyses that have been made are in close agreement, 

 and other formulae have been proposed. Doubtless each of the above 

 kinds has a definite chemical composition, and the variations shown 

 by the different analyses are possibly due to the fact that two — (a) and 

 {b) or (h) and (c) — of the three kinds may occur together in the same 

 crystal, as observed by Bertrand and by mj'self in specimens from 

 Leadhills. It will therefore be necessary to examine optically each 

 fragment that is collected for future analyses of leadhillite. 



III. — Gigantic Irish Deer-Remains. — Report of the Committee, 

 consisting of Professor W. Boyd Dawkins (Chairman), His 

 Honour Deemster Gill, the Rev. E. B. Savage, Mr. G. W. 

 Lamplugh, and Mr. P. M. C. Kermode (Secretary), appointed 

 to examine the Conditions under which remains of the Gigantic 

 Irish Deer are found in the Isle of Man.^ 



WE were able to add a footnote to our report of last year to the 

 effect that a fairly perfect skeleton of Cervus giganteus had 

 been discovered, of which we hoped to hand in details with this 

 year's report. These remains were found in a marl -pit at Close-y- 



1 Communicated to the British Association, Bristol, September, 1898, Section C 

 (Geology). 



