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Figure 5.3 John Strachey's 1725 cross-section through the Earth showing
dipping strata caused by its rotation (from B. D. Webby, Proceedings of
the Geologists' Association 80 (1969), 91-97, Plate 5).
quenched subterranean fires thought to be located beneath the Earth's
crust. This head of steam exerted pressure on overlying strata which
triggered the shockwaves.
Others attempted to explain why strata dipped. Not all sedimen-
tary beds are horizontal; indeed, it is common to find inclined strata,
which we now attribute to the effects of tectonic movement of the
Earth's crust. In the 1700s no one knew that earth movements could
occur on a scale large enough to move lithospheric plates or conti-
nents. Inclined strata were explained away by invoking several ideas:
John Ray noted that coal miners thought that beds dipped towards the
centre of the Earth; and the English mining surveyor John Strachey
(1671-1743) suggested in 1725, in a paper published in the
Philosophical Transactions of the Royal Society, that strata became
formed and separated from each other (and became inclined) because
of the rotational effect of the Earth (Figure
5.3
), an idea modified from
