Geological Relations of the East of Ireland. 147 
§ 29. This mica slate, to the east of the glen, is connected with 
Scar-hill, and to the west, it forms the hill called Brocka, being 
the southern termination of Tonelagee range. In its farther progress 
to the south-west, it traverses Glenasane near the head of the glen, 
forms the lower and southern part of Comaderry mountain, crosses 
Glendolough, and penetrates through Lugduff, into, and across 
Glenmalur. In all these glens, in which the mica slate and granite 
have been broken through, we perceive a perfect parallelism in the 
disposition of the mica slate on either side, being inclined in its line 
of apposition to the granite, at an angle of 20° to 25°. 
§ 30. About one hundred yards below the ridge of the lower 
part of Comaderry on the southern side, in a line opposite to St. 
Kevin’s bed, are masses of trap, occupying the brow of the hill for 
about two hundred yards in width, and forming in the upper part 
a nearly continuous rock, but in the lower occurring only in accu- 
mulated or scattered blocks, occasionally intermingled with fragments 
of granite. The higher part consists mostly of common hornblende 
rock, which varies in colour from green to black, and approaches 
even to the nature of massive mica. About the middle of the 
declivity, it is intermixed with felspar, mostly compact, and greyish 
white, the two substances constituting a coarse-grained rock ; at times 
the felspar greatly predominates, containing hornblende and mica 
disseminated in its substance. Lower down the brow, the rock 
consists of a compact felspar base with prismatic crystals of horn- 
blende, interlaced, and shooting through the felspar in every di- 
rection, thus forming a beautiful assemblage. The hornblende 
crystals sometimes appear in stelliform groups, and the prisms extend 
from one to three inches in length. The exterior coating of the 
hornblende rock is variously affected by weathering ; and in one 
instance, the colour of a tabular mass, about two or three inches in 
T 2 
