THE AGE OF BRONZE 
tangled masses of savage thorns surmounted the wall to 
make it still more difficult to scale. 
As a further step men found that, by ramming down 
successive layers of earth between retaining frames of 
planks, they could make the mound much stronger and 
more durable. Sometimes they embedded logs and 
bundles of sticks in the clay to give it additional rein- 
forcement. The original Great Wall of China, built 
about 210 B.c., but now almost entirely destroyed, was 
constructed in this way. 
Finally someone hit on the expedient of making the clay 
into separate bricks and drying them in the sun, like the 
adobe bricks still so much used in our own Southwest. 
It seems curious, in view of the fact that the baking of 
pottery had long been known, that the hardening of bricks 
by baking them in kilns instead of by merely drying them 
in the sun should have remained so long undiscovered. 
People undoubtedly used unshaped stones for various 
purposes, including that of building, even before the in- 
vention of bricks. Already in the New Stone Age, in some 
regions, huge rough stones had come to be used in various 
ways, especially in connection with the worship of the 
dead. Circles, avenues, and monuments, composed of 
single standing stones, sometimes of gigantic size, occur 
in various parts of the world, along with dolmens—tombs 
constructed of three or more upright stones supporting 
another which formed the ceiling of the funeral chamber. 
A great mound of earth usually if not always covered the 
dolmens, though this in some cases has since disappeared. 
The most famous of all these stone constructions is the 
“circle” of Stonehenge in England, which seems to have 
been connected in some way with the worship of the sun 
and to date from the Bronze Age. Here some of the 
stones, weighing many tons, had been transported long 
distances and shaped, dressed down, and provided with 
sockets and tenons for securing the capstones, involving 
an enormous amount of work. But we know that even in 
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