on my mind. Whilst examining an old gravel-pit near
Shrewsbury, a labourer told me that he had found in it a large worn
tropical Volute shell, such as may be seen on the chimney-pieces of
cottages; and as he would not sell the shell, I was convinced that he
had really found it in the pit. I told Sedgwick of the fact, and he at
once said (no doubt truly) that it must have been thrown away by some
one into the pit; but then added, if really embedded there it would be
the greatest misfortune to geology, as it would overthrow all that
we know about the superficial deposits of the Midland Counties. These
gravel-beds belong in fact to the glacial period, and in after years I
found in them broken arctic shells. But I was then utterly astonished at
Sedgwick not being delighted at so wonderful a fact as a tropical shell
being found near the surface in the middle of England. Nothing
before had ever made me thoroughly realise, though I had read various
scientific books, that science consists in grouping facts so that
general laws or conclusions may be drawn from them.

Next morning we started for Llangollen, Conway, Bangor, and Capel Curig.
This tour was of decided use in teaching me a little how to make out the
geology of a country. Sedgwick often sent me on a line parallel to
his, telling me to bring back specimens of the rocks and to mark the
stratification on a map. I have little doubt that he did this for my
good, as I was too ignorant to have aided him. On this tour I had a
striking instance of how easy it is to overlook phenomena, however
conspicuous, before they have been observed by any one. We spent many
hours in Cwm Idwal, examining all the rocks with extreme care, as
Sedgwick was anxious to find fossils in them; but neither of us saw
a trace of the wonderful glacial phenomena all around us; we did not
notice the plainly scored rocks, the perched boulders, the lateral and
terminal moraines. Yet these phenomena are so conspicuous that, as
I declared in a paper published many years afterwards in the
'Philosophical Magazine' ('Philosophical Magazine,' 1842.), a house
burnt down by fire did not tell its story more plainly than did this
valley. If it had still been filled by a glacier, the phenomena would
have been less distinct than they now are.

At Capel Curig I left Sedgwick and went in a straight line by compass
and map across the mountains to Barmouth, never following any track
unless it coincided with my course. I t