Geology Reference
In-Depth Information
eroding off his bare, plowed fields by enclosing them behind stone walls. Stacking blocks
of sandstone quarried from nearby hills, Hutton couldn't help but recognize the similarity
between the mineral grains leaving his fields and those that composed the rocks he piled.
There, in his hands, below his feet, and before his eyes, lay the keys to a grand cycle
in which rocks eroded and the resulting sediment was deposited elsewhere and buried
deep enough to reform into new rock. Most rocks in Britain are made of sediments
eroded from somewhere else, and everywhere above sea level is eroding. Neither idea was
new—Leonardo had long before recognized the nature of sedimentary rocks, and most
farmers were familiar with erosion. But Hutton did something new: he put these ideas to-
gether, seeing them as two halves of a grand cycle. Here was the foundational insight be-
hind his radically original concept of deep time.
Such a cycle presented a dilemma. Without a way to restore eroded material, the soil
would eventually disappear and, along with it, the fertility of the land, something a bene-
volent creator would not allow. What could refresh the land after erosion wore it down?
After setting up his farm, Hutton moved back to Edinburgh in 1767. He arrived in a
city on the cusp of an intellectual renaissance. The Scottish aristocracy that backed Bon-
nie Prince Charlie's failed attempt to claim the throne had been purged, dismantling class
distinctions and ushering in a new egalitarian spirit that fostered innovative thought. The
new intellectual culture that sprang from the ruins of Edinburgh society nurtured Hutton's
curiosity and interests.
At the time, most natural philosophers thought rocks precipitated out of Werner's drying
primeval ocean in a global version of those grow-your-own crystal sets. But Hutton's con-
tinual experimentation with mineral chemistry convinced him that rocks contained a lot
of material that would not dissolve in water. How could rocks precipitate out of a drying
sea if they could never be dissolved in the first place? And if Werner's conventional wis-
dom about how minerals formed was wrong, then what could be responsible for solidifying
rocks? Hutton theorized that the combined effects of heat and pressure offered the only vi-
able alternative. Both would be available at the bottom of a pile of sediment—as long as
the pile was thick enough.
In 1784 the newly chartered Royal Society of Edinburgh invited Hutton, then nearly
sixty, to present his theory of the Earth, forcing him to gather his thoughts into presentable
