Geological research focused on two distinct activities for the first half of the 19th century. There were
those concerned with the description and classification of rocks, minerals and fossils and, most notably, the
subdivision of Earth history. British geologists such as Sir Roderick Murchison and Adam Sedgewick were at
the forefront of this campaign, with European counterparts such as Alcide d’Orbigny not far behind.
Other researchers were concerned with the geological processes operating on and within the Earth, and how
these processes may have operated in the geological past. In other words, how rocks came to be formed
and subsequently deformed. Foremost amongst these was Sir Charles Lyell, who considered himself on a
crusade to make geology scientific. Observations led to theories about how geological processes operated.
In Lyell’s view, these processes were gradualistic — a steady state Earth in which geological processes were
the same in the past (“Uniformitarianism”, following on from ideas earlier expressed by Hutton). By contrast,
many geologists in continental Europe favoured the theory promoted by Georges Cuvier that the Earth had
experienced a more eventful past, with catastrophes punctuating Earth history, these events being associated
with tectonics, extinctions and major changes in deposition (“Catastrophism”). The debate of the importance
of Uniformitarianism versus Catastrophism continued throughout much of the 19th century and persists in
some circles even today; although most geologists are now happy to accept that Earth history is a response
to a combination of both gradual and sudden processes.
However, many 19th century geologists (as today) were both describers/classifiers and interpreters,
attempting to add colour to the pages of Earth history by envisaging past worlds. What did a Jurassic Earth
look like and what creatures inhabited it? Which geological processes were operating to leave us with the
rock record we see today? Such intriguing questions are equally valid nowadays and engage the imagination
of most geologists to a greater or lesser extent, even if their focus is often on the fine detail. The romance
of imagining our past Earth is something that still draws students to study geology and requires both an
understanding of geological classification and of geological processes.
By the second half of the 19th century, much of the basic classification work had been completed (although
this continues to the present day in order to provide ever-increasing precision) and greater numbers of
geological scholars were focused on interpreting the rocks they studied in terms of the processes responsible
for their creation and deformation. Such studies ranged from the small-scale, for example, Henry Sorby
and his interpretation of rocks in microscopic thin-sections, to the large scale, such as Eduard Suess and
his interpretation of the formation of mountain belts. Although Western Europe continued to be a hub for
geological research, American researchers, such as James Dana and Louis Agassiz, were now also making
important contributions.
Geologists, however, were still faced with the perplexing conundrum — how old was the Earth? It was widely
accepted to be millions of years in duration, but exactly how many remained an unknown. The discovery of
radioactivity, as the 19th century passed into the 20th century, provided the breakthrough. Radioactive decay
of elements present in certain rocks could be measured and interpreted in terms of absolute age. At last,
there was a clock of Earth history!