O'Reilly — On a Cylindrical Mass of Basalt, 23'3 



XXXIX. — On a Cylindrical Mass of Basalt existing at Contham 

 Head, Moon Bay, Coast oe Anteim. By Joseph P. O'Reilly, 

 C. E., Central School of Paris ; Professor of Mining and Mine- 

 ralogy, Royal College of Science for Ireland. Plates 8 and 9. 



[Eead May 12, 1879.] 



"When engaged in studying the forms of columnar basalt at the Giant's 

 Causeway, I had occasion to observe and to take sketches of a very 

 remarkable cylindrical form of that rock, which occurs at the N. W. 

 point of Moon Bay, marked Contham Head on the Ordnance Map, and 

 situated at about two miles distance east of the Giant's Causeway. 



When first observed by me in 1875, I was quite unable to explain 

 its origin and connexion with the containing rock ; but having subse- 

 quently revisited the locality, for its further examination, and the 

 making of a few sketches, I was enabled to arrive at certain conclu- 

 sions as to the nature and origin of this remarkable form, which I beg 

 leave to submit to the Academy. 



Contham Head is a small promontory or spit of basaltic rock, ex- 

 tending out into the sea about 250 feet, its breadth at the water level 

 being about 200 feet. There is a central rib rising above the under- 

 lying basalt bed, about twenty to thirty feet at most, having a length 

 of about 150 feet, and a breadth of about fifty feet. 



This low, narrow, and relatively short promontory is only acces- 

 sible from the cliffs by a narrow and precipitate pathway, used prin- 

 cipally by the kelp-gatherers, but decidedly uninviting for those not 

 accustomed to such ground. Such a point, quite out of the usual 

 track of tourists, is but rarely visited, and I should not have had any 

 knowledge of it had I not chanced to observe the remarkable cylin- 

 drical form when passing quite close to the shore in a boat, catching 

 thus the bold outline presented by the eastern side of the rock form. 



The cylindrical mass, known among the boatmen as the " mill- 

 stone,'''' occurs at the point of the promontory where the narrow spit 

 joins the mainland. It here extends across the spit in an E.-W. di- 

 rection, having a horizontal length of about fifty to sixty feet. 



The section is not quite circular, being rather oval. The greater 

 diameter, the horizontal one, is about twenty-four to twenty-five feet, 

 and the vertical one about eighteen to twenty feet. It presents a 

 series of apparently concentric joints, irregularly distributed as regards 

 distance from the centre, and is furthermore broken up by radial joint- 

 ing, also irregularly placed, and giving rise to blocks of forms very simi- 

 lar to those observable at the Causeway. The mass rests upon and is 

 partially enclosed by an amygdaloidal basalt of the rich violet colour so 

 remarkable in many of the beds along the coast, and characterized by 

 the presence of masses of scarlet jasper wherever in contact with the 

 sea water. This amygdaloidal basalt is much corroded, and is being 



