The rock used to make the General Post Office is sandstone that came
from Ireland. Sandstone is a sedimentary rock made of condescend masses
of sand deposited by moving water or by wind. The chemical construction
of sandstone is made up of quartz. The material that binds, or glues,
the rock together is usually made of silica, calcium carbonate, or
iron oxide. The color of the rock is usually determined by the cementing
material and other materials in the rock.
The general rocks found in this area (Ireland) are connemara marble
which is a metamorphic rock from old limestone from 600 million years
ago. Serpentine gives connemara marble its green color. Granite is
another rock found in Ireland most of it is about 400 million years
old. Devonian sandstones (old red sandstones) dating from 345-395
million years ago are other rocks found here. Carboniferous rocks
dating from 280 to 345 million years ago is the main surface rock
type and covers more than 50% of the country. The Last main rock found
in the area is Teriary Basalt dating from only about 40 million years
ago.