OF THE MEXAl BASIN. 
237 
greywacke, quartz, and micaceous rocks, are observed, as 
well as on the sides of some others of the higher mountains 
of' this range. The rocks in these places have less of the 
slaty vertical structure, which is so predominant elsewhere. 
They often assume, when the basalt prevails, a columnar 
form. Near the summit of Snowdon, in the hard grey- 
wacke, impressions of shells are found, proving, in the clear- 
est manner, that all the rocks of this chain belong to the 
transition-formation. 
In cutting a new road along the western side of the 
lower Llanberris lake, fine sections of the hard rocks are 
exposed to view. W e can there trace veins of asbestus, 
from an inch to three or four in thickness, passing through 
a tough asbestine greenish slate rock ; and in other places 
of this road, in the sections so formed, are great blocks of 
amygdaloid, and coarse conglomerate puddingstone rock, 
which, from the extreme hardness of the cementing matter, 
and from the beauty of some of the imbedded pebbles, 
might probably be sawed and polished into ornamental 
slabs for tables. 
Two fine examples of immense masses of basalt are af- 
forded in the hills of Penmaen Bach and Penmaen Maur. 
The cap or summit of the latter in appearance resembles 
much a volcanic crater. It is curious to observe, that the 
north-west front of this last hill is the favourite resort of 
multitudes of the common house-swallow, whose clayey 
nest covers in many places the rock ; 
— " This guest of summer, 
The temple-haunting Martlet, does approve, 
By his loved mansionry, that the heaven's breath 
Smells wooingly here : no jutty frieze, 
Buttress, nor coigne of vantage, but this bird 
Hath made his pendent bed and procreant cradle. 
Where they most breed and haunt, I have observed. 
The air is delicate." Shakspeare. 
