434 E. T. Hardman — White Chalk of Tyrone. 



two rows of cell-apertures on the celluliferous face, but it differs in 

 possessing a median keel. It agrees with Fenestella in the possession 

 of the latter, but differs fi'om both it and Polypora in the total absence 

 of non-celluliferous dissepiments or cross-bars, and in having the re- 

 verse keeled in a similar manner to the obverse or celluliferous face. 

 Localities. — Braidwood, Carluke : in the Fenestella bed of the 1st 

 Calmy Limestone (Lower Limestone Group). Gair Quarrj^, near 

 Carluke ; in shale over the Gair Limestone (Upper Limestone 

 Group). It has also been obtained from other localities in Lanark- 

 shire by the collectors of the Geological Survey. These will be 

 given in the forthcoming Explanation to Sheet 23, Scotland. 



EXPLANATION OF PLATE XV. 



Fig. 1. Cannella cellulif era, tv,'ice the natuval size. 

 „ 2. Celluliferous face of the same, six times the natural size. 

 „ 3. Reverse of the same, showing keel and striae, six times the natural size. 

 ' For these very excellent drawings I am indehted to my friend, Mr. H. Skae. 



n. — On Analysis of White Chalk from the County of Tyrone, 

 WITH Note on the Occurrence of Zinc therein, and in the 



OVERLYING BaSALT.^ 



By Edward T. Hardman, F.E.G.S.L; of the Geological Survey of Ireland ; 

 Associate of the Eoyal College of Science, Dublin. 



I WAS led to make this analysis with the view of determining if 

 possible whether the extreme hardness of the Irish Chalk were 

 due to either a chemical, a mechanical, or a calorific alteration, from 

 the influence of the overlying basalt. If it were owing to chemical 

 change, we should expect to find a large per-centage of silicates and 

 a diminution in the amount of lime ; if to the influence of heat, 

 carbonic acid would be driven off, silica would be in excess, and the 

 presence of the insoluble bases, such as the peroxide of iron, oxide of 

 manganese, and alumina, would become more apparent; while if a 

 mechanical cause or pressure were the reason, no change would take 

 place in the relative amount of the constituents. I believe the result 

 seems to show that the induration of the Chalk must be set down 

 chiefly to the last agency, — if to anything apart from the original 

 formation of the rock, — and that the power of alteration exerted over 

 it by the heat of the molten basalt has been small indeed. At the 

 same time a certain addition has been made to it by means of water 

 holding chemical bodies in solution. 



The following is the result of the analysis. The specimens used 

 were obtained from an old quarry in the townland of Legmurn, 

 about a mile and a half north-east of Stewartstown. The Chalk is 

 so indurated as to be in reality a hard splintery limestone. 



, , ( Very perceptible even in small 

 ■ ( quantities of the Chalk. 

 ... a trace. 

 ... a trace. 



CaCOg ... 



.. 97-320 



ZnO 



M0GO3 ... 



.. 0-890 





SiO, 



.. 0-537 



BaO 



AI263 



.. 0-273 



SrO 



FejOs 



.. 0-095 



K3O 



FeO 



a trace. 



Na^O 



Amount not estimated. 

 1 Bead before the Royal Geological Society of Ireland, June 11, 1873. 



