[ 43 ] 



VII.— ON THE CALCITE CEYSTALS PEOM THE lEON MEA- 

 STJEES OF THE COUNTY ANTEIM. By PEOEESSOE 

 J. P. O'EEILLY, C.E. (Plate 11.) 



[Eead, February 19, 1883.] 



In the very interesting Paper read before the Natural Science sec- 

 tion of the Eoyal Dublin Society, April 11th, 1881, by Mr. Philip 

 Argall, on the " Tertiary Iron Ore Measures of Grlenariff Yalley, 

 Co. Antrim,"^ he makes frequent mention of the " aragonite crys- 

 tals" which accompany the ores and are found in geodes or vugs 

 of the roof of the ore bed, or in cells of the amygdaloidal dolerite 

 along with zeolites, as also at other points of the deposit in fine 

 acicular crystals. He remarks, p. 158 : " Hence a display of crys- 

 tals (aragonite) on the roof is regarded by some as a sign of poverty; 

 similarly as a display of spar crystals in a standing vein indicates 

 poverty of mineral matter in the percolating fluids when the run 

 was being filled." At page 161 he says, "when approaching a 

 dyke, cracks in the pavement are frequently filled with acicular 

 crystals of aragonite." 



Mr. Argall very kindly procured for me the linings of a couple 

 of fine geodes, and in the accompaning letter explained by a sketch 

 the position of the vugs or cavities relative to the ore bed in which 

 the crystals are found. Subsequently, in October last, Professor 

 Hull handed me for the Museum collection of the Eoyal College 

 of Science some specimens of acicular crystals from the same 

 contact found at Cushendal, Co. Antrim. I had previously been 

 engaged in examining the crystals of the vugs forwarded me by 

 Mr. Argall, and from the densities obtained had reason to consider 

 these crystals as being really calcspars. A further examination of 

 the acicular crystals presented by Professor Hull lead me to the 

 same conclusion in their regard, and I now beg leave to submit the 

 results of this examination from the points of view of density, 

 crystalline forms, cleavage, and hardness. 



^ Proceedings, vol. iii. (ne\7 series) part iv., p. 151, 



