1908. The British Association in Dublin. 235 



THE LATKRITK AND BAUXITE ZONE OF NORTH-EAvST 



IRELAND. 



EV PROl'KSSOR GRKNVII,I,K A. J. COLE, F.G.S. 



This paper was merely explanatory of au exhibit of the types of rock 

 formed during the interval between the basaltic eruptions in the North 

 of Ireland in Eocene times. It was urged, in agreement with the views 

 of Richardson and Tate, and Holden, that the red lateritic zone repre- 

 sents basalt altered in situ even down to depths of forty feet, the so- 

 called " volcanic bombs " in the layer being residual lumps of less 

 altered basalt. Such a tj'pe of alteration is clearly connected with the 

 climatic conditions of Eocene times. Some of the pisolitic iron-ore may 

 have accumulated on the surface of the laterite in pools formed during the 

 rainy seasons. The pale bauxites are derived from sporadic eruptions of 

 rhyolite, and the bi-pyramidal crystals of quartz in them prove this over 

 a wide area. The thin bauxitic layer, occurring as it does above the 

 pisolitic iron-ore, may be in part formed by wind-borne material. 



NOTE ON THE OCCURRENCE OF (SO-CALLED) CAVE PEARLS. 



BY HAROI<D BRODERICK, M.A. 



Cave pearls, as thej- were first called in " Cave Hunting" (Professor 

 Boyd Dawk ins), seem to be of comparatively rare occurrence. They con- 

 sist of a nucleus of some foreign material, frequently a small pebble of 

 Yoredale rock (in one case a small fragment of lead ore), coated by 

 numerous concentric rings of calcite. All those found have been 

 formed in what might be called nests in the rock, into which drops 

 of water have fallen at comparatively long intervals from a considerable 

 height. Each falling drop will have the tendency to slightly turn the 

 nucleus, and also, by deposition, to coat it with a thin film of carbonate 

 of calcium ; this deposition is continued until what is called a cave pearl 

 is formed, ranging in diameter from "5 cm. up to 2 cm. 



The three types with which I am acquainted come. from three separate 

 caves: i. The Blue John (Derbyshire). This type consists of a nucleus 

 of Yoredale Sandstone covered with layers of calcite, which become 

 harder towards the outside, the exterior being extremely hard, smooth, 

 and opaque; sp.gr. 275. 2. The Bagshawe (Derbyshire). This type 

 consists in man}' cases of a nucleus of Yoredale Sandstone, or, in one 

 case at least, of lead ore ; the concentric deposit in this type is some- 

 what translucent, the outer surface is slightly crystalline; sp. gr. 2*71. 

 3. Marble Arch (Co. Fermanagh). This type has a nucleus of Yoredale 

 Sandstone, while the covering is composed of carbonate which seems 

 to have included in it a considerable admixture of contained mud the 

 colour, is a dirty grey, and the deposit is comparatively soft ; sp. gr. 2*40« 

 As will be seen from the specific gravity in each case, the deposit is in 

 the form of calcite, a condition which might be anticipated from the 

 mode of occurrence. 



