1903 - 4 .] Mr J. G. Goodchild on Intrusive Rocks. 
211 
of one’s fingers into a book, referred to above in connection with 
sills. References to the letterpress of almost any text-books on 
Geology will suffice to show that this relationship is what the 
authors had in mind when they wrote. Strangely enough the 
figures of dykes in these books are usually drawn in accordance 
with the facts, just as figures are which relate to sills or to other 
forms of intrusive rocks. 
Out of a large number of cases a few will suffice to show that 
dykes generally replace their own volume of the rocks they invade. 
This is the case, just as it is with sills, quite irrespective of either 
the lithological character or the structure of either the intruder 
or the country rock. Fig. IS is traced from a photograph show- 
ing the upward termination of a Tertiary basalt dyke in New Red 
Sandstone, near the Borough Cemetery at Belfast, and figs. 16 and 
17 other dykes traversing Chalk at Whitewell Quarry, Belfast. 
These show an entire want of correspondence between the opposite 
walls of the country rock, such as could not have occurred had 
the dykes filled simple rents. For both of these I am indebted to 
Miss Andrews. Fig. 1 1 is taken from a photograph by Mr Voge, 
showing the upward termination of a similar dyke in Chalk 
at the White Rocks, near Portrush. The rounded patch seen 
above the end of the dyke is probably the continuation of the 
same dyke, which has bent in its upward course 6 , so that it passes 
behind the face of the cliff for a short distance. Fig. 19 shows 
a tertiary basalt dyke, which ends off abruptly in a remarkable 
melange of (Devonian) granite and Highland Schist at Torr na 
Sealga, in the Ross of Mull, already referred to. This locality will 
be referred to presently in another connection. Again, in the cliffs 
formed by the basalt lavas of Skye and Mull, many fine examples 
of the same kind are clearly laid open to view. This is especially 
the case in the grand range of precipices forming the cliff below 
Beinn an Aonidh, on the south shore of Mull, west of Carsaig. 
There may be seen dykes and sills of basic rocks which zigzag 
their way up the face of the cliff through the various beds of lava 
without producing the least disturbance of these volcanic rocks, 
and without adding their own thickness to that of the pile in 
which they occur. Fig. 20 shows some intrusions at Carsaig 
Arches, sketched from the sea. 
