410 
REV. S. HAITGHTON ON THE JOINT-SYSTEMS OE IRELAND 
Table, is that between Waterford and Cornwall. If we compare together the primary 
and secondary joints in each locality, we find the following Table: — 
Table XI. — Angle between Primary and Secondary Joints. 
Waterford. 
Donegal. 
Mourne. 
Cornwall. 
Fermanagh. 
Primary (A, C) and First Secondary (A', C') 
+ 
27° 5' 
+ 
32° 24' 
+ 
31° 46' 
+ 
+ 
31° 1' 
Primary (A, C) and Second Secondary (A", C") 
37° 11' 
30° 56' 
27° 28' 
This Table discloses a very interesting and unexpected result, viz. that, in Waterford, 
Donegal, Mourne, and Fermanagh, the angle between the Primary and First Secondary 
Joint-Systems ranges between the narrow limits of 27° 5' and 32° 24', and that in 
Waterford, Mourne, and Cornwall the angle between the Primary and Second Secondary 
Joint-Systems ranges from 27° 28' to 37° 11'. 
I hope to be able to show that this important result of observation is an easy con- 
sequence of the mechanical theory of Joints; but before doing so, I shall prove by the 
following Table, that the theory of Conjugate Joints used in my paper on the co. 
Waterford applies equally well to the other districts examined by me. 
Table XII. — Angles between Conjugate Joints, measured from East to North. 
Systems. 
Waterford. 
Donegal. 
Mourne. 
Cornwall. 
Fermanagh. 
Primary (A, C) 
Secondary (A 1 , C') 
Secondary (A". C"'! 
89° 11' 
91° 52' 
91° 20' 
93° 19' 
CO QO 
O 00 
CN 
© 
90° 21' 
92° 30' 
94° 18' 
91° 20' 
The tendency of Conjugate Joints to place themselves at right angles is plainly shown 
by the foregoing Table. 
Paet VI.— MECHANICAL THEORY OF ROCK JOINTS. 
Rock masses are always arranged in sheets, whose dimensions in two rectangular 
directions are much greater than in the third direction at right angles to the first two. 
Hence, when a system of forces acts upon such a mass, its first effect will be to produce 
a system of fissures at right angles to the plane of the resultant force. When other 
forces are subsequently applied to the rock mass, already divided into bands by the 
parallel fissures, their effect upon these rock bands will be different according as they 
have a large vertical or large horizontal component. In the first case the bands of rock 
will simply bend and break across along new fissures at right angles to the original 
fissures. This cause produces the phenomena of conjugate joints, which have been 
described and accounted for by several writers, and have been fully discussed by myself 
in the paper, “ On the Physical Structure of the Old Red Sandstone of Waterford,” so 
often referred to by me. 
