In the historical development of geology, there were conflicting ideas about the formation of crystalline rocks (the rocks we today refer to as igneous or metamorphic). In the late 1700s, an influential German mineralogist named Abraham Gottlob Werner: (1749-1817) argued that crystalline sediments chemically precipitated from an initial universal ocean early in Earth’s history. While he published little, and frail health precluded traveling beyond his local area, Werner was a very influential professor at the Freiberg Mining Academy where he infected students with an almost disciple-like fervor. This concept of rocks precipitating from ocean water became known as Neptunism.
Another famous early geologist was James Hutton
(1726 –1797), a wealthy Scottish polymath who dabbled in medicine, chemical
manufacturing, agriculture, and geology. Often called the “Father of Modern
Geology”, he wrote Theory of the Earth in 1788 where he argued for the
principle of uniformitarianism – the idea that the same processes that operate
on Earth today were the same that have operated throughout Earth history. He
also advocated that crystalline rocks formed gradually over time from the
cooling and crystallization of molten rock or magma. This idea was in direct
opposition to Werner’s idea of Neptunism and came to be known as Plutonism
setting the stage for a vociferous debate between these two ideas in geology in the late 1700s.
Meanwhile,
in France, a geologist named Nicolas Desmarest (1725-1815) was studying and mapping
rocks in Auvergne, a mountainous area in central France. While his passion was
geology, Desmarest supported himself with a government job as Inspector General
and Director of Manufactures which enabled him to travel around the country,
often on foot so he could examine the local rocks.
In Auvergne, Desmarest found columnar basalt lava flows
which he was able to trace back to volcanic-like craters in the region. It was
clear that the basalt lava was erupted from these extinct craters. He presented
a geologic map of the area to the Paris Academy of Sciences which was later published
in 1777. His results clearly supported the ideas of Plutonism.
When Desmarest was approached by those who wished to debate
Neptunism vs. Plutonism, he was reputed to have said “Go and see.” This quote,
which appears in most introductory textbooks in historical geology, strongly
resonates with modern geologists. Theorizing about the Earth is all well and
good, but if your ideas don’t stand up to field checking, there’s little point
debating it.
That’s why we have field trips in geology. It’s one thing to tell students that the Hudson Valley was once covered with a warm, shallow sea – a fact that they’ll dutifully write in their notebooks without questioning. It’s another to take them to a limestone rock outcrop on the side of a local highway and show them the abundant marine invertebrate fossils in the rock. Want to learn about the geology of the Hudson Valley? Go and see – look at the rocks and let them speak to you.
