Aug., 1889. 
PETROLOGY OF LOCAL PEBBLES. 
175 
ago, the physical conditions in which, judging by the analogy 
of what is going on at the present time, these beds accumu¬ 
lated : he has pictured to us the mountain torrents bringing 
down their loads of stones from the interior of the great 
mountain ranges, and depositing them where they emerge 
from the mountains on to plains, where the fall of the 
ground was insufficient to produce the current necessary to 
carry the burden further. In this transportation it is plain that 
a very powerful process of selection would go on. The harder 
rocks would evidently be those which would be the fittest to 
survive, and we all know that these pebbles are about as 
tough and intractable as any stones we have to do with. 
Now the quartzite composing the pebbles is certainly in 
great part very different from any rock which at present 
appears at the surface in the Midlands, or indeed for very long 
distances from us. We have quartzite formations at the Lickey, 
Nuneaton, the Wrekin, and further off in the borderland of 
Shropshire and Wales; but, so far as I know, these would 
only furnish one of the types of rock which we find in the 
pebble beds. Of the other types which, from their structure, 
cannot. I think, have been mere local variations in a quartzite 
mass, the present exposures afford no examples. 
The origin of the pebbles—the direction in which they 
have been brought into the district—has been vigorously 
discussed, and is at the present time quite unsettled. 
Professor Bonney has minutely examined the formation 
principally in the area of Cannock Chase, and believes 
that great rivers or shore currents brought the stones from 
the North, even finding parallels for some of the rocks in the 
Torridon sandstone of the far Northern Highlands. It has 
been objected to this view that the pebble beds attain their 
maximum of thickness and of richness in pebbles about the 
district of Cannock Chase, where they reach a thickness of 
300 feet; while, as they are traced further north towards 
Nottinghamshire on the one hand, or Liverpool on the other, 
the pebbles become fewer and ultimately disappear, the rock 
becoming first a pebbly sandstone, then a sandstone with 
occasional pebbles. Further north still, in Yorkshire, they 
are altogether wanting, the lower mottled sandstone being the 
only representative of the Bunter, while at Carlisle the Keuper 
beds are the only representatives of the Triassic strata. 
I have taken the above summary from the important paper 
read by Mr. W. J. Harrison to the Philosophical Society in 
this city in 1882, on the derivation of the quartzite pebbles 
of the Drift and the Triassic strata. In this paper he shows 
reasons for believing that the pebbles came from the west. 
