O'Eeilly^ — On Lines of Jointing of Bray Head, 8fc. 235 
sedimentary origin and nature whicli np to the present has been 
assigned to them by geologists. 
The reasons which have tended to force this view upon me may 
be classed under the two broad heads, Physical characteristics, and 
Chemical composition. As, however, the further development of these 
reasons is not a necessary part of the subject of the present Paper, I 
propose to treat the question as fully as possible in a further Paper, 
in which will be embodied my Eeport on the results of the Chemical 
Analyses of the Eocks of Eray Head, for the execution of which the 
Academy has made me grants out of the yearly sum set apart for 
Scientific research. 
In the following detail of directions (the numeration of which is 
continued from Part IL of this Paper), the bearings are given east or 
west of true north, and were reduced from magnetic bearings, the varia- 
tion being taken from the Admiralty Chart of the Bay of Dublin, which 
gives for 1882 the variation as 22° 5' W., decreasing at the rate of 8' 
per annum ; in 1886, it was 21° 23' W. 
[Detail of Observed Directions, etc. 
