(_a_) The Sarsen Stones are the remains of a cap of Tertiary Sandstone
which once covered the plain. (See page 17.)
(_b_) The Foreign Stones are still a matter of debate. They have
assuredly been brought from a distance. This is unusual; megalithic
structures are usually built of materials found close at hand. (See
page 20.)
[Illustration: Stonehenge. Looking towards the South East.]