260 A. McHenry — TrachjHc Rocks of Antrim. 



the odd molecule of magnesia set free on the right-hand side of the 

 equation being disregarded ; and, since pseudoraorphs of serpentine 

 after olivine generally retain the precise form and dimensions of the 

 crystals they replace, I conjectured that the surplus of 30 per cent, 

 might be partly represented by the veins of chrysotile, which so 

 frequently traverse serpentinous rocks. 



Zirkel, in his great work,^ discusses the subject, and points to an 

 experiment of K. Miillei-,^ w^ho found that under the action of water 

 containing carbonic acid powdered olivine lost a part of its silica 

 and magnesia, and almost all its ferrous oxide, while the insoluble 

 residue consisted of silica and bases in approximately the same 

 proportion as in serpentine. The same distinguished author also 

 suggests that the magnesia liberated from olivine may assist in the 

 serpentinization of bronzite. I had already indicated that this might 

 be possible (p. 110, op. cit.), but at the same time pointed out that 

 the increase of volume in this case must be even greater than when 

 olivine alone is concerned, as is shown in the following case : — 



Olivine. Enstatite. "Water. Serpentine. 



Molecules ... 4+2 + 6 = 3 



Volume 170 + 64-5 + 108 = 330 



whence 23'i-5 volumes of olivine and enstatite give rise by 

 hydration to 330 volumes of serpentine, an increase of 40 per cent. 

 It will he observed that the volume of the water which enters into 

 combination is neglected ; if this be taken into account it will be 

 found that in accordance with the general rule the volume of the 

 final product is less than the sum of the volumes of its constituents. 

 A consideration of such changes as we have indicated is of interest 

 from more than one point of view. The marked increase in the 

 volume of the solid substances produced by hydration will lead us 

 to look for the secondary minerals which must have been deposited 

 in association with serpentine rock ; while the large quantity of 

 water absorbed in the reaction may well impress us with the 

 important part played by the hydration of minerals in removing 

 water from the liquid envelope and locking it up in the solid form 

 within the earth's crust. 



VII. — On the Age of the Tbaohttio Eocks of Antrim. 



By Alex. McHeney, M.R.I. A. ; Geological Survey of Ireland. 



[Communicated by permission of the Director- General of the Geological Survey.] 



SINCE the publication of the Geological Survey Maps and 

 Memoirs of Antrim, some additional evidence has been 

 obtained bearing on the age of the trachyte of that district. In the 

 year 1888, when engaged in travei'sing a line of section running 

 from Belfast in a northerly direction across county Antrim to the 

 Giants' Causeway, I was fortunate enough to notice in a large 

 chalk quarry at Templepatrick railway station evidence which 



1 Lehrbuch der Petrograpbie, 1894, p. 390. 

 - Min. Mittb. 1877, p. 36. 



