i6 HIStORr of the SOCIETr. 



it ; the planes wliich feparate the prifms paffing equally through 

 the ffiells and the ftone itfelf. 



The grain of this flone paffes by infenfible fliades from a 

 high degree of finenefs, until it become undiflinguifliable from 

 that of the common columnar bafaltes. 



The name of Siliceous Bafalt, which DrRiCHARDSON employs, 

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

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

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

 tion ofjilica than ufual. 



The ftrata of filiceous bafalt, both at Portruih and the Skerry 

 iflands, generally alternate with ftrata of equal thicknefs of a 

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

 ftrata 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 ftra- 

 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 J yet the line of demarcation is left completely dif- 

 tind. 



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

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

 thefe ftrata. 



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

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

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

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

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

 each other alternately ; this is fucceeded by an accumulation 

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

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

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

 Here commences an accumulation of many ftrata of the filiceous 

 bafalt alone, which ftretches along the fliore for about 100 

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

 nues 



