tidal aestuary, there is geology enough to be learnt, to explain the
greater part of the making of all the continents on the globe.



II. THE PEBBLES IN THE STREET



If you, dear reader, dwell in any northern town, you will almost
certainly see paving courts and alleys, and sometimes--to the
discomfort of your feet--whole streets, or set up as bournestones at
corners, or laid in heaps to be broken up for road-metal, certain
round pebbles, usually dark brown or speckled gray, and exceedingly
tough and hard. Some of them will be very large--boulders of several
feet in diameter. If you move from town to town, from the north of
Scotland as far down as Essex on the east, or as far down as
Shrewsbury and Wolverhampton (at least) on the west, you will still
find these pebbles, but fewer and smaller as you go south. It
matters not what the rocks and soils of the country round may be.
However much they may differ, these pebbles will be, on the whole,
the same everywhere.

But if your town be south of the valley of the Thames, you will find,
as far as I am aware, no such pebbles there. The gravels round you
will be made up entirely of rolled chalk flints, and bits of beds
immediately above or below the chalk. The blocks of "Sarsden"
sandstone--those of which Stonehenge is built--and the "plum-pudding
stones" which are sometimes found with them, have no kindred with the
northern pebbles. They belong to beds above the chalk.