O'Eeilly — On a Cylindrical Mass of Basalt. 241 



That there have been many oscillatory vertical movements is more 

 than probable, and every fact tending to prove those oscillations is of 

 interest geologically. 



I naturally sought for further confirmation of my views as regards 

 the mode of formation of those cylindrical forms and their connexion 

 Avith the basalt dykes, and I am under the impression that the bold 

 mass of rock which rises from the sea at the entrance of Portroon 

 Creek, half a mile west of the Giant's Causeway, and which I have 

 sketched, affords such a confirmation. The dyke is perfectly distinct, 

 Avith its transverse columnar formation, and presents, in my opinion, 

 a whorl or cylindrical mass immediately in connexion with it. The 

 distinctness of the form is not, however, so evident as to allow of my 

 comparing it with the mass at Contham Head, but merits, however, 

 being recorded as a term of comparison. 



