HISTORT of the SOCIETT. 



MINERALOGY. 



The Reverend Dr William Richardson, late F. T. C. D., 1803. 



having fent to Dr Hope a colledlion of fpecimens from the north- Rcmiiks oa'iu^ 



em coaft of Antrim, v;ith a catalogue, and obfervatlons, the fpe- coaaof a1 

 cimens were exhibited, and the obfervations were read in the 

 Royal Society, March 1803. 



SILICEOUS BASALT. . 



Dr Richardson difcovered the foffil to which he gives thiis 

 name in the peninfula of Portrufli, four or five years ago. It 

 abounds alfo in the Skerry iflands, a reef of rocky iflots extend- 

 ing from the northern point of Portrulh-head for about a 

 mile eaftward. A fmall part of every one of thofe iflots is form- 

 ed of this ftone, while the remainder confifts of coarfe bafalt, 

 fimilar in all refpedts to that on the eaft fide of the above-men- 

 tioned peninfula. It is met with in one or two pther places. 



This ftone is arranged in ftrata, from ten to twenty inches 

 thick, all fteadily parallel to one another, and every ftratum, as 

 far as can be obferved, preferving an uniform thicknefs through 

 its whole extent. When thefe ftrata are quarried into, they ap- 

 pear to be conftrucfted of large prifms, generally pentagonal, 

 which when broken divide into fmaller prifms. This internal 

 prifmatic conftrudiion frequently gives an irregular or fhivery 

 appearance to the fradiure, which however is often conchoidal, 

 and the grain as uniform as in the Giant's Caufeway bafaltes. 



The beds of this foflil are remarkable for containing marine 

 exuviae in great abundance, particularly imprellions of coniua 

 ammonis. The flat fhells and impreffions contained in thefe 

 ftones are fteadily parallel to each other, and perpendicular to 

 the axis of the prifms. It muft be obferved, that the prifmaric 

 conftrud:ion is never interrupted by the fhells difperfed through 



Vol. V.-^P. III. G it ; 



