352 [B.N.F.O. 



DESCRIPTION OF PLATE. 



Figure I. — Typical specimen of chalcedony from Carnmoney 

 Neck, showing the characteristic reniform or spherical 

 crystallisation of the mineral ; the section through the 

 spherical concretion to the right shows the central 

 crystal of calcite invested by a zone of white zeolitic 

 chalcedony. 



Figure II. — Exterior surface of chalcedony from Carnmoney, 

 showing the hollow pseudomorphous cavities formerly 

 occupied by calcite and zeolites. 



Figure III. — Typical vein of banded chalcedony from, Carn- 

 money, showing a portion of the rock with the usual 

 thin layer of calcite and hullite between the former 

 and the latter. This structure is most clearly seen, 

 where a small quantity of hullite is almost completely 

 surrounded by the chalcedony. The thin white layer 

 between the dark hullite and the chalcedony is com- 

 posed of calcite. 



AUTHORITIES AND REFERENCES. 



Bischof, Prof. G., 1854. Fehrbuch d. Chem. Geol. 1 — Referred 

 to on the growth of quartz from solution. Vol. III., 

 p. 636. 



Cole, Prof. G. A. J., 1895. "Hullite."— Proc. Belfast Nat. 

 Field Club. Series II. Vol. IV., pp. 221-225. 



Gault, W., 1879. 'O n tn e mode of occurrence and probable 

 origin of the Hullite and other siliceous minerals 

 found in the volcanic neck of Carnmoney,' etc. — 

 Proc. Belfast Nat. Field Club. Series II. Vol. I., 

 PP- 353-357- 



Geikie, Sir A., 1897. 'The Ancient Volcanoes of Great 

 Britain.' Vol. II., pp. 272-3. 



