320 A. Tercy Ilosldns — Glauconite from Antrim. 



siliceous organisms are few, probably accounts, to a large extent, for 

 the variation in silica. 



It is just possible that at Woodburn the alkalies may be introduced 

 into minerals capable of retaining them in combination, through the 

 percolation of waters from the large salt deposits of Carrickfergus. 



It may be interesting here to notice the figures given in Column III, 

 from the volume of the " Challenger " Report referred to above, for 

 a reddened glauconite in which the silica is exceptionally low. 

 To this analysis the following remark is attached : " The high 

 percentage of ferric oxide and water points to a decomposition of 

 this mineral, which has been transformed into limonite, as is often 

 the case in glauconite from the geological strata, with loss of silicic 

 acid and potash, but this explanation can hardly be given for this 

 specimen, which consists of casts from a coral sand." But no 

 alternative suggestion is made, and neither will the suggestion hold 

 in the case of our own glauconite; for, while the silica is low, 

 neither the ferric oxide nor the water is particularly high, while 

 the potash is seemingly higher than usual. Moreover, the casts are 

 green, and show no signs whatever of decomposition into limonite. 

 Piofessor Cole has suggested that possibly in the course of ages 

 silica has actually been abstracted in solution ; but in this case, 

 since the silica must have been in combination when present in the 

 mineral the grains should show some signs of the decomposition 

 which must have taken place, and no such signs are visible. The 

 only conclusion to be arrived at in the present state of our informa- 

 tion is that the glauconite from Woodburn, when originally formed, 

 w^as a variety containing an unusually small percentage of silica. 



I trust I may have a further opportunity of examining specimens 

 of glauconite from this locality, with a view to determining 

 whether the material is constant in composition in different zones. 



In conclusion I wish to express my sincere thanks for the kind 

 assistance I have received from Miss S. M. Thompson, of Macedon, 

 Belfast, and Professor Grrenville A. J. Cole, of the Koyal College of 

 Science, Dublin, in the preparation of this note. 



