History Weblecture for Unit 40
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We move back in time somewhat to start on our next topic, which includes both the establishment of modern theories of the formation of the earth and modern theories of the origin and development of forms of life. Both these theories have serious implications for the religious views held by many people, and both biology and geology have been the source of controversies inside and outside scientific discussions.
As early as Aristotle, natural philosophers had discussed rocks that appeared to be imprinted with teeth, leaves, bones, and other plant or animal remains. Aristotle thought the origins of these fossils were inorganic, growing like crystals in the earth, a not unlikely premise given that many fossils represent animals no longer found on earth. Robert Hooke was able to show through microscopic observation that petrified wood and fossil shells too closely resembled their living counterparts to be a separate kind of object. Hooke concluded in his Micrographia that petrified shells had once been living sea creatures, somehow transformed by chemical deposits from "petrifying water" into stone. By the end of the 18th century, it was common for a well-educated person to have his own collection of petrified stones and shells, as King George III did with his collection, now in the British Museum.
George III's fossil collection from the British Museum Exhibit on the Enlightenment, 29 June 2007
©2007 Christe Ann McMenomy
Nicolaus Steno was a Danish naturalist who worked for the Grand Duke of Tuscany in Italy. Steno was influenced by Hooke's ideas and also by the corpuscular theory of matter to interpret certain stony objects found in mud flats as shark's teeth, fossilized by Hooke's process. He published a study of the rocks and minerals in the mountains near Florence, and produced three guiding principles for the study of the earth:
Colorado Monument, Grand Junction, CO.
Steno became a Catholic and left geology to serve as Apostolic Vicar of Northern Germany; in his later life he devoted his time to theology. He believed that the world was relatively young, and that fossil-bearing strata had been deposited by the Biblical Flood.
Read about Nicolas Steno at the Berkeley history of science site.
The Flood figured prominently in most theories throughout the eighteenth century, as few European scientists and philosophers were willing to completely abandon Biblical accounts of creation and the development of the earth. However, there was increasing controversy over when the flood had occurred, and how extensive it was. Georges Buffon was among the first to propose an expanded timescale, a seven epoch division over about 75,000 years, instead of the Biblical 6 days of Creation. He based his time scale on calculations made by Newton, and determined the time necessary for an earth-size sphere of iron to cool from red hot to its current average temperature. Buffon's epochs were
|Earth forms out of matter ejected by comet collision with sun (hence the molten starting point).|
|Crust cools, forms mountains.|
|Vapors condense, cause flood.|
|Further cooling causes cracks; water sinks into earth's interior and dry lands appear.|
|Land animals appear.|
Buffon had no evidence to back his theory, but it stimulated interest among a number of naturalists, who began making field observations. The growing prosperity of the middle class and the rapid growth of trade made it possible for many to voyage across the seas and even around the world. These newly rich patrons of the arts and sciences did not have an aristocratic tradition, but they did have the leisure and money to educate themselves and their children in the "new learning".
Two major groups developed, with the "Neptunists" promoting water as the primary force of geological formation and the "Vulcanists" promoting heat.
In the eighteenth century, the German naturalists studied the sedimentation of rocks, and realized that many currently dry areas would once have been covered by seas. Abraham Werner proposed that rock strata formed from a universal primeval ocean. The process produced four kinds of rock: primitive rocks crystalized out of the ocean (without fossils), transitional rocks resulting from chemical precipitation, sedimentary rocks (rich in fossils), and derivative rocks such as sand and clay, which formed from weathering of the other three kinds.
At the same time, French naturalists realized that basalt flows were the result of volcanic activity, and linked many mountains and basalt outcroppings to ancient volcanoes. Basalt often appears as giant crystals, similar to those formed during slow cooling of similar rocks.
A third theory developed which stressed the geological activity of the internal heat of the earth (in contrast to the surface heat favored by the Vulcanists). The "Plutonists" believed that geological forces operated at the same rate and in the same way throughout the world for its entire existence. Studying the current processes was equivalent to studying the general mode of past processes. The foremost Plutonist was the Scottish geologist James Hutton. Hutton believed that the interior of the world was molten rock which pushed into cracks in the crust, forcing sedimentary strata up (and tilting it in the process). This harder rock survived as the core of the mountain when the sedimentary exterior while eroded by wind and water.
Read this short summary of Hutton's theories at the University of Minnesota's SHiPS center.
The chief opponent to Hutton's "uniformitarian principle", as the theory of constant geological processes was called, was the Frenchman Georges Cuvier, who countered it with a theory of "catastrophism" based on discontinuities in the fossil record. Cuvier thought that a series of floods had occurred: a few significant single events, rather than one long, continuous process. The most recent of Cuvier's flood would coincide with the one recorded in the Bible. Because of its alignment with the Biblical Flood stories, Cuvier's theory was well-received.
By the beginning of the nineteenth century, geologists were a recognized group of specialist scientists. They had developed a methodology for studying and identifying different strata layers in many parts of Europe. The English surveyor William Smith published his classification of rocks in 1799, along with a detailed description of the area around Bath, in England. The Cambridge professor of geology, Adam Sedgwick, studied the rocks in Wales, and Sedgwick's friend Roderick Murchison studied the rocks in Devon. The Welsh sediment series (called Cambrian from the ancient name for Wales) appeared to be much older than the Devonian sediments, which themselves contained fossils of many fish. Both names are preserved in the current geological time scale.
Lyell revived Hutton's theory, and in 1833, published The Principles of Geology, in which he tried to explain the processes of the past in terms of those he could observe. Lyell insisted that geological forces had been constant throughout the history of the earth. He reclassified a series of fossils into the Eocene, Miocene, and Pliocene series, all within the Tertiary series identified earlier by students of Steno. Lyell's efforts resulted in the classification of geological strata into the Precambrian, Paleozoic, Mesozoic, and Cenozoic eras which is still in use (see Science lecture).
Permanent glaciers, north side of Mt. Rainier, WA
© 2009 Christe Ann McMenomy
The Swiss-American Agassiz was fascinated by glaciers. He studied the glacial action of ice in the Alps, and proposed that a glacier must once have stretched across the whole of central Europe. He performed similar researches after he became a professor at Harvard University in Boston, and showed that the same evidence existed for glacial action across most of North America. From the rates of motion of the current glaciers, Agassiz determined that at least some geological processes were sporadic -- occurring for brief (geologically speaking) periods of time, then quiet for long periods of time. Agassiz's theories countered the uniformitarianism of Lyell and softened the catastrophism of Cuvier.
Read the short summary of Agassiz's work at the Berkeley Museum of Paleontology site.
In the midst of the growing debate over the age of the earth, based on different models of geological processes, Darwin wrote The Origin of the Species. We will talk about this more in the next few weeks, but Darwin's theories challenged the geologist to come up with a method of accounting for the isolation of one species or form of life (such as marsupials in Australia), while accounting for the spread of others across continents now separated by wide oceans.
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