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The GEOLOGY of PUFFIN ISLAND. 
By J. W. Grecory, F.G.S., 
GEOLOGICAL DEPARTMENT, BRITISH MUSEUM. 
[Read 10th December, 1887.] 
PuFFIN ISLAND is a knoll of Carboniferous limestone, 
lying a mile from the easternmost point of Anglesey, and 
forming part of the north shore of the Menai Straits. 
The island is three quarters of a mile long by a quarter 
broad, its longer axis lying N.E. and §.W., the backbone 
consisting of a ridge, at one place 195 feet high, traversing 
the island close to the south shore, above which it rises as 
a steep cliff, while to the north it extends as a gentle dip 
slope. As it is composed throughout of this one formation 
its geological structure is very simple, though it presents 
several problems of interest of which as yet no complete 
solution can be offered. 
The Carboniferous limestone is one of the best known 
rocks in the kingdom, partly from its extensive use as 
an ornamental stone known as encrinital or Derbyshire 
marble, but mainly from its prominence in so many of our 
most tourist-haunted landscapes, such as the gorge of 
Avon, the pass of Cheddar, the dales of Derbyshire, the 
screes of Llangollen, and the caverns of the Pennines and 
the Peak. It was deposited in the comparatively shallow 
and clear sea that succeeded the lakes of the old Red Sand- 
stone and preceded the forests of the Coal measures. The 
sea seems to have been quiet, and the paucity of mechani- 
cally derived sediment indicates that the hmestone was 
deposited at some distance from land, the rock being 
composed almost entirely of the remains of crinoids, 
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