16 HISTORY of the SOCILTT. 



it ; the planes which feparate the prifms pafling equally through 

 the fhells and the {tone itfelf. 



The grain of this flone paffes by infenfible fhades from a 

 high degree of finenefs, until it become undiftinguifhable from 

 that of the common columnar bafaltes. 



The name of Siliceous Bafalt, which DrRiCHARDSON employs, 

 was firft given to this foffil by Mr Pictet of Geneva, when he 

 vifited Portrufh, in a tour through Ireland two years ago. He 

 confidered it as a variety of bafalt, containing a greater propor- 

 tion offtlica than ufual. 



The flrata of filiceous bafalt, both at Portrufh and the Skerry 

 iilands, generally alternate with flrata of equal thicknefs of a 

 coarfe-grained bafalt of a grey colour. The materials of the 

 flrata grow into each other, fo as to form one folid mafs, from 

 which it is eafy to quarry pieces in the confine of the two flra- 

 ta, with a part of each adhering ; but the coarfe bafalt, as it ap- 

 proaches very near to the fine, always abates fomewhat of its 

 coarfenefs ; yet the line of demarcation is left completely dif- 

 tind. 



The peninfula of Portrufh lies about fix miles to the weft of 

 the Giant's Caufeway, and on its eaflern furface alone prefents 

 thefe flrata. 



In the fpace of about 700 yards, it exhibits in miniature thofc 

 changes and interruptions of the flrata, which occur on the large 

 fcale along the northern bafaltic coaft of Ireland. At the place 

 where it emerges from the flrand, there firft occurs a mafs com- 

 pofed of flrata of the coarfe and filiceous bafalt, placed over 

 each other alternately ; this is fucceeded by an accumulation 

 of regular flrata of the coarfe bafalt alone. - A fecond alterna- 

 tion, and a fecond accumulation of the coarfe-grained flrata, 

 come in order, and extend to the well called Tubber Wherry. 

 Here commences an accumulation of many flrata of the filiceous 

 bafalt alone, which flretches along the fhore for about 100 

 yards, and then changes into a third alternation, which conti- 

 nues 



