476 W. J. SOLLAS ON THE SILURIAN DISTRICT 
streams into a number of long hilly ranges running north and south. 
The streams are the river Taff, the river Rhymney, and a small feeder 
of the latter, which, from one of its branches, we may designate as 
the Nant Mawr. The hilly ranges consist wholly of Old Red Sand- 
stone, except at their southern terminations, which are composed of 
the Silurian rocks under description. The Silurian strata east of 
the Rhymney extend northwards from the Rhymney bridge to a line 
joining the village of Rhymney on the west with that of Cross 
Dowton on the east. Between the Rhymney and Nant Mawr 
the Silurian composes the hills south of a line which commences 
west of Cefn Coed, continues parallel to and north of the stream 
flowing past Llwyn-y-grant-uchaf, and ends in the road leading 
from Pen-y-lan to Lanedern. Between Nant Mawr and the Taff 
the country south of Heath Farm is completely covered over by 
coarse gravels and shingles, which conceal every trace of the under- 
lying rocks except at one or two points where they have been cut 
through by the Rhymney railway. Silurian rocks are thus exposed 
. as far north as the spot marked Starting House on the Ordnance 
Fig. 1.—Geological Sketch Map of the Cardiff Silurian District. 
(Scale, 1 inch to a mile.) 
=n Old red pany Scortan Dark band Alhpaal or Siiarians 
FI cla ymney Gri. Graval deposits _ nat exposed &c. 
map; and there can be little doubt that they form the southern half 
at least of the hill on which Heath Farm is situated, and probably 
extend still further west. A careful search for them in the 
embankment at Llandaff railway-station revealed, however, nothing 
but river-gravels, made up chiefly of Old-Red and Carboniferous 
fragments, with an occasional pebble of doubtful Silurian age. 
The three districts thus indicated constitute together a single and 
continuous area, which extends from the Starting House on the west 
to a little beyond Cross Dowton on the east (a distance of about 
