Page 69 - Handbook of Gold Exploration and Evaluation

50 Handbook of gold exploration and evaluation

Bernoulli Principle, which is central to the analysis of fluid flow. This principle
allows the total energy at any point in a streamline to be written as a constant in
terms of fluid density, fluid velocity, total pressure at the point, and the elevation
of that point relative to an arbitrary datum (see Chapter 4). Hutton's Theory of
the Earth (1788) presented firm geological evidence of a sedimentary cycle of
erosion. His grasp of the immensity of geological time provided the key to
understanding the formation and denudation of landscapes and the uplift of
sediment deposited as mud and sand on the seafloor during the denudation of
previous landscapes. His thesis, The Present is the Key to the Past proposed
immeasurably long periods of time for great thicknesses of sediment to
accumulate and suggested that the history of the Earth could be explained by
what is happening now.
Although geology had by this time entered a new period of growth, the
significance of Hutton's observations was largely overshadowed by a continued
acceptance of Neptunian and Plutonistic concepts. Werner, a German scientist
and a leading theorist of Neptunism, still argued that volcanoes were local
phenomena caused by burning coal seams and that all rocks that would ever
exist had already been formed and ± that continents were slowly being washed
into the sea and would eventually disappear. Plutonists, as represented by
Hutton, thought that the Earth's heat would cause sediments to rise from the sea.
In 1788, Hutton identified some rock formations, which appeared to consist of
sedimentary rock that had been metamorphosed by heat.
But the seeds of understanding had been sown. In 1802, Playfair elaborated
and expanded on Hutton's idea of small changes taking place over immense
periods of geological time. In a publication Huttonian Theory of the Earth he
asserted that `no valley is independent of the rivers flowing in it, but instead,
develops various characteristics governed by its geology and the environment
within which it is placed'.
By this time, scientists such as Nicholas Desmarest, a French geologist, had
shown that the basalt rocks of the Auvergne region of France were created from
lava. At Moscow University, Professor Shchurovsky (1803±1884) deduced that
`plutonic rocks lifting the Urals enriched them in gold and other valuable
minerals'. He also recognised the length and complexity of magmatic processes
associated with mountain building and ore formation and the fracturing that
subsequently led to the formation of vast detrital deposits in particular beds of
stratigraphic successions. Extensive gold-bearing gravels discovered in 1829 in
the Lena Basin, Eastern Siberia are still being worked today. Apart from the
Witwatersrand lithified conglomerate deposits of South Africa, they are
probably the largest gold deposits ever found in an alluvial setting.
William Smith, an English civil engineer, may have been the first person to
use fossils to map rock strata. During the late 1700s, while surveying and
building canals in southern England, Smith had examined layers of rock