1822.] Geology of Snowdon, and the surrounding Country. 415 
vation applies to rocks of the Pass of Pont Aberglaslyn, where 
the same rocks and slates are manifestly interstratified. Just 
beyond Pont Aberglaslyn, the road to Tan y Bwlch divides; 
the new road being more even than the old one, which traverses 
the elevated and barren mountain region between the two 
places, we walked over the latter as probably affording the 
best opportunity of observing the nature of the rocks. For 
several miles we found nothing different from those every where 
observed, except that, generally speaking, they are harder, and. 
the slates more generally inchixe to blue. In one instance, 
we perceived a rock perfectly resembling those of the ruin 
covering the side of Penmanmawr. A short distance before we 
began to descend towards Tan y Bwlch, however, a change 
was perceived; ridges of rock in which no steatite was ob- 
served were interstratified with the slates parallel to their 
cleavage plane, and these running in the direction of NE 
and SW, and apparently consisting only of fine grained 
ehlorite, calcareous spar, and quartz, the latter prevailing ; 
one ridge appeared to consist of granular quartz including 
a very few specks of chlorite. These rocks rose in ridges 
eonsiderably above the slates bounding them on each side, 
and with them dipping towards the NW. at the usual angle 
of 54°. 
Slates prevail on both sides of the vale of Festeniog below 
Tan y Bwlch Inn towards the sea, the flat base of the vale 
consisting apparently of alluvial matter: the beauty of this vale,. 
however, which is justly celebrated, appears to be owing chiefly 
to the decomposition apparent in the slates wherever they are 
visible, forming a soil in which the woods of its northern bank. 
flourish luxuriantly. A part of the hill on the opposite side is 
clothed in like manner, but the trees are of less height, the soil 
in which they grow being also a slaty rock in part decomposed, 
or in fragments generally so very small that we could not perceive 
the direction of the cleavage plane. On the summit of the hill, 
however, just before we began the descent to a waterfall termed 
the Rhyader Dha, about two miles from Tan y Bwlch, the rock 
did not differ from those of the base of Moel Shabod, and is 
interstratified with slates : a fallen mass appeared in the descent 
to the waterfall consisting of the same rock emclosing round or 
ovate masses of a quartzose substance. In the basin of the 
upper waterfall, we observed a new appearance in the arrange- 
ment of the rocks so often noticed. The slates here appeared 
in nearly a horizontal position, and exhibited a tortuous course, 
which we have never before observed, while the rocks lying 
between. slates assumed a somewhat columnar form, thus : 
