56 
PHOCEEDINGS OF SOCIETIES. 
with rounded pebbles of grey quartz, is found resting on the igneous 
rock at an inclination of 15°. The division of the two rocks is here far 
better defined than at the junction near the to wn, where there appears to 
be a passage from the sandstone into the syenite. A stream bears along 
during floods the quartzy pebbles set free by disintegration from the 
conglomerate ; and it is interesting to contemplate the shingle that once 
rolled on the shores of an ancient sea, now hurried on in the bed of 
the mountain torrent after its rest of ages. The sandstone here is pro¬ 
bably the extremity of a narrow band of that rock that extends under 
the waters of Loch Corrib to Cong, and rises near that town with a 
thick covering of its own detritus. 
A junction of a different kind may be examined about three miles 
south from Oughterard, where the beautiful syenitic porphyry abuts 
against a hornblendic rock, that forms with it a range of hills varying 
from 600 to above 900 feet in height; from whose summits the greater 
part of the igneous district of West Galway may be seen, stretching away 
to the ocean with its sombre covering of heathy moor, and drearily 
speckled with the leaden tints of its hundred lakes and pools. 
I have attempted to show the features of the junction at Oughterard 
in a plan and diagram section:— 
No. 1 is the ordinary limestone, but changing its character at its 
junction with the sandstone, where it assumes an arenaceous appearance, 
and becomes full of crystals of calcite. 
No. 2 is a bed of yellow sandstone, six feet thick. 
No. 3 is also abed of sandstone, thinly laminated, and full of minute 
crystals of pyrites, three feet thick. 
No. 4 is a bed of quartzose sandstone, four feet thick. 
No. 5 is a remarkable rock. It may be called a sandstone conglo¬ 
merate : but on the surface I found some four-sided pyramids, from two 
to three inches in height, and more or less perfectly shaped like crystals. 
These are mineralogically different from the rock itself, and somewhat 
from each other, assuming in various degrees a syenitic character. They 
are spotted with pyrites, and contain small crystals, apparently of horn¬ 
blende. This rock is interesting in a double point of view, as it illus¬ 
trates the passage of a sandstone into a syenite, and shows a tendency of 
the latter to form crystalline shapes, which, in this example, enclose 
smaller crystals of its contained minerals. The pyramids, and, indeed, 
all the granitic rocks, are covered with a coat of shining black, that 
reminds one of the incrustations which Humboldt says are found on 
granites washed by rivers of the torrid zone. 
No. 6 is a rock of very varied appearance. In some places it might 
be called quartz rock; in others it resembles a conglomerate, and in 
others again it approaches a syenite, into which it finally seems to pass. 
The inclination of the stratified rocks increases from 30° at the head 
of the lower waterfall to 60° at the summit of the upper, and it again 
decreases to 30° at the junction with the syenite ; and it is worthy of 
remark that the planes of the principal joints of the latter at No. 7 
display a conformability with the bedding of the former. 
