Geology Reference
In-Depth Information
infested with monsters, the air filled with flying dragons.
Deserts appeared and disappeared creating wind-blown sand
dunes and ephemeral lakes that evaporated into beds of salt. In
tropical swamps the decaying vegetation of countless forests
slowly turned into coal. Grey limestones became grey cemeter-
ies for entombed shells and corals, the land once more
submerged beneath a warm temperate sea; again retreating,
again advancing, until earthquakes shattered the rocks as a vast
ocean closed; continents collided pushing up the sea bed into
massive mountains in Snowdonia. It was a dramatic picture.
Holmes told the story of Kelvin and his battle with the
geologists. He derided the hour-glass methods, showing how
present erosion rates are much faster now than in the past. He
blamed this on Man, for his poor management of our
'greatest
economic asset'
the soil, and for the burning of coal and oil,
which
'has for many years steadily increased the potency of
the atmosphere to rot away the rocks'
. Finally, he arrived at
radioactivity - the new and elegant way of measuring geologi-
cal time.
Describing how radioactive minerals had become the geolo-
gist's timekeeper, natural clocks that kept a material record of
the passing years, he told how the oldest minerals so far dated
were one thousand seven hundred and fifty
million
years old, and
how meteorites had been accumulating helium for seven
thousand
million years. Time was deep, time was gigantic, time
was eternal. But what, exactly, did all it mean for the age of the
Earth?
The exploration of the past is still incomplete, for there still
remain wide-spread areas of ancient rocks of undetermined
ages. At any moment the present age record may be sur-
passed by new discoveries. All we can be sure of is that our
planet has already existed for well over 1750 million years,
and probably for not less than 2000 million years - a million
times longer, that is, than the whole of the Christian Era. The
birth-time of the earth has receded into an inconceivable
