Oct., 1889. 
PETROLOGY OF LOCAL PEBBLES. 
241 
Some less determinable pebbles occur in all the various 
pits, but require much more examination. 
I liad hoped to have found in the nature of the pebbles 
some criterion by which the drift pebbles might be distin¬ 
guished from those of the Triassic pebble beds. The data 
for such a discrimination are, I fear, not vet sufficiently 
numerous to make an opinion worth very much, but I think 
that the presence in a bed of the grey felsites, or pebbles of 
the type of the Wrekin rocks, would afford strong pre¬ 
sumption of later age. 
The striking feature of the Triassic pebbles is the 
abundance of tourmaline in several different forms of rock. 
Dr. Lapwortli tells me that there is a marked absence of 
tourmaline in the chain of the Scandinavian Mountains, as 
well as in its British extension in Northern Scotland, while 
we know that tourmaline rocks are verv abundant in Cornwall 
«/ 
and Devon, and also in the remains of the Great Southern 
Mountain Chain more to the east, with which the rocks of 
those counties appear connected. It will be remembered that 
in the series of Norwegian rocks brought home by Mr. Pumplirey 
this summer, there were none containing any tourmaline, or, 
at least, none in which it was an essential constituent. 
It appears, threefore, as if we might on the whole look to 
the great southern ancient range of mountains for the origin 
of the Triassic pebbles, but there is nothing to show from how 
far south the rivers came which brought them down. The 
absence, or at least very great rarity of granite, would 
indicate that at that time the deeper-seated portions were 
not yet bared by denudation. On the other hand, we have 
specimens of the felsites which so frequently form off-slioots 
from granite masses, and from their micro-granitic texture we 
may infer that we have more than the most superficial parts 
of the felsites. The quartzites also would naturally occur 
more on the outer flanks of the range rather than in the 
interior parts. On the other hand, the crushed and 
laminated condition of some of the rocks shows that the 
processes of elevation and crumpling were well advanced; 
that we have not, as it were, the washings of the earliest stages 
of the mountain-making period. 
The pebbles mixed with this southern type in the upper 
gravel beds are much more distinctly of the character of the 
Welsh and lake country rocks. Granite is more frequent, 
and slate and gritty ashes occur which are quite wanting in 
the undoubtedly Bunter deposits. In addition, there is the 
marked difference between the types of felsites which has been 
noted above. 
