A Short History of Historical Geology
Prior to the 1600's it was not recognized that the Earth had a history that was more extensive than that outlined in the book of Genesis in the Old Testament of the Judeo-Christian Bible.
It was generally assumed that the Earth had changed very little in any substantive way since it was created some 4000 years earlier.
Events in through the 1600's and 1700's would begin to change these views. During this period of time people began to study the Earth and address the question of its origins and history in ways that we would characterize as 'scientific'.
Nicholas Steno
Perhaps the most significant event in geology in the 1600's occurred one day in October of 1666 when some Italian fishermen landed a large shark in Tuscany. The Grande Duke of Tuscany ordered the head of the shark delivered to Florence to be dissected by an anatomist named Niels Stensen (better known by his Latinized name Stenonis, or Steno).
It was in dissecting the head of the shark that Steno noticed the resemblance between the teeth of the shark and a type of rock called a 'tongue-stone'. Although recognized today as the fossilized teeth of large sharks, tongue-stones were not believed to be organic in Steno's time.
Steno became convinced that tongue-stones were shark teeth, and he set out to prove this by demonstrating that tongue stones could not have formed in the rock, but rather, had to have been entombed after coming out of a shark. He did this by demonstrating that tongue-stones were already worn when they came out of the rock, and thus could not have been formed in-place.
Steno argued that the earth which enclosed the tongue-stones must have originally been soft due to its mixture with water, for the tongue-stones impressed their shape on the rock, and that the layered appearance of the rock must be due to sediments settling out of water, either at the time of creation or during the great flood.
Thus, a consideration of tongue-stones and shark teeth led an anatomist to consider the problem of how to explain how one solid (a shark tooth) gets inside of another solid (a rock).
In 1669, Steno published a broad theory explaining how objects could become entombed in sedimentary rock because the rock formed after the production of the object.
Steno's researches established three basic principles of historical geology.
The 1600's and 1700's saw continued expansion of the population of Europe as well as the expansion of building and manufacturing that would explode with the early 1800's with the industrial revolution.
The Neptunists
Abraham Gottlob Werner
One of the most important mining regions in Europe at the time was located in Saxony (part of modern northern Germany). Around this mining region grew a tradition of studying the minerals and layers of the Earth that eventually produced the Mining Academy at Freiberg. It was here in the late 1700's that Abraham Gottlob Werner came to study minerals and mining, eventually becoming the most famous professor of geological subjects in Europe.
Werner was a brilliant and charismatic lecturer who trained a generation of students and sent them out into the world to study its rocks. From his studies in this area, Werner developed a theory of the Earth, which, although now known to be wrong, laid out the steps and principles of its formation in a way that others could judge and test by examining rocks and rock layers in the field.
Werner was a Neptunist meaning that he believed that the outer part of the Earth observable by people (the crust) had formed from crystallizing out of the waters of the ocean that covered the Earth after it had formed.
In Werner's time the three modern rock types - igneous, metamorphic, and sedimentary were not yet fully recognized. Instead, Werner and his contemporaries recognized the following sequence:
Alluvium (relatively modern sands and gravels)
Volcanic (lava, ash, etc)
Secondary and tertiary (sedimentary rocks)
Transitional (older, hard sedimentary rocks)
Primary or Primative (crystalline igneous and metamorphic rocks)
Werner noted that the primary rocks were crystalline and formed the deepest layers as well as the cores of mountainous areas. Werner believed that the original, underlying nucleus of the Earth was uneven and that as the primary rocks crystallized they coated the high spots of the nucleus to form mountains.
The most controversial aspect of Werner's theory was that he believed that volcanic rock was not a significant portion of the Earth and that volcanoes were localized phenomenon caused by the melting of rock due to burning underground coal seams. Werner did not believe in igneous rocks or igneous processes in the Earth's interior.
The Fall of the Neptunian Theory
Ironically, it was Werner's success as a teacher that led to the undoing of his theory of the Earth. Several of Werner's most devoted students went on to become renowned geologists. Each encountered great difficulties in applying the Neptunian theory to the geology that they encountered outside of northern Germany.
The most apparent problem with the Neptunian theory became obvious when Werner's students began to study volcanoes in Italy and France. In these places they found many types of crystalline primary rocks that were unmistakably products of volcanic eruptions. Furthermore, in Italy, near the active Mt. Vesuvius there were no known coal deposits that could be burning to supply the heat of the volcano. The volcanic fires appeared to be coming from the interior of the Earth itself.
The Plutonists/Vulcanist
The Neptunist theory of the Earth was not the only game in town. At the same time the Werner was expounding on the primordial ocean and primary rocks, other students of the Earth were entertaining a different set of ideas. The Plutonists/Vulcanist held that both fire and water had played major roles in the formation of the Earth's rocks. To generalize, the Plutonists/Vulcanist believed that the interior of the Earth was hot and that volcanoes were openings or vents into this fiery interior. The heat of the Earth served to fuse and sometimes melt the accumulated rocky debris that collected in low spots on the Earth's surface and the fiery exhalations of the Earth had the force to push rock up, deforming and contorting the crust into mountains.
James Hutton
The leader of the Plutonist/Vulcanist school was an educated gentleman farmer who gave up farming in 1768 to devote himself to studying the geology of Scotland. It was outside of Edinburgh where Hutton made his now famous observation of an angular unconformity.
Hutton observed that layers of rock were tilted at a steep angle and that above these tilted layers lay flat lying layers of younger rock (younger by Steno's principle of superposition). Hutton argued this implied that at some time in the past the lower layers were deposited (formed as originally horizontal layers) and then later upturned in some cataclysmic upheaval. After this they must have been worn flat at then had the younger layers deposited on top.
Hutton realized that a sequence of events like this would have taken immense amounts of time to unfold. The fact that Hutton had observed many other sequences like this in various parts of Europe suggested to him that the Earth was very old and that its surface was wearing down and then being built up by "revolutions" from the interior. Writing about the age of the Earth, Hutton said, "we find no vestige of a beginning - no prospect of an end." Hutton invented the concept of Deep Time.
By the early1800’s plutonism/vulcanism was clearly making the most converts. Not only were Werner's own students abandoning neptunism, but several Scottish colleagues of Hutton were working to apply his ideas. These included Sir James Hall and John Playfair, who helped popularize Hutton's ideas by writing a companion book to Hutton's own Theory of the Earth that helped to rescue Hutton's ideas from his own dense and turgid prose.
The Great Uniformitarian
The ideas of Hutton, Hall, and Playfair were greatly influential to a young English geologist working in the early 1800's. Charles Lyell (born in 1797 - the same year that Hutton died) traveled extensively in Europe and North America and was able to show that geologic features formed in the past could be explained by process of erosion, deposition, and volcanism seen operating in the present.
Lyell argued that it was unnecessary to assume that violent or unusual events had occurred in the past that were not occurring today (for example, a great Noachian flood - which was used by many nineteenth century geologists to explain a variety of landscape features). Lyell called this principle uniformitarianism and held to it strictly.
In the 1830's Lyell published a 3 volume work called Principles of Geology which outlined the philosophy of uniformitarianism and presented a carefully argued methodology for interpreting geology. It was enormously influential and widely read.
Paleontology - Extinction and the Succession of Lost Worlds
William Smith
Smith was born on March 23, 1769, into a family of small farmers. He received little formal education, but from an early age took an interest in exploring and collecting fossils in his native Oxfordshire in England. At the same time, he learned geometry, surveying, and mapping; at the age of eighteen he became an assistant surveyor, learning his trade from the master surveyor Edward Webb. Surveying required Smith to travel all over England; in 1794 he toured the entire country, and then he began to supervise the digging of the Somerset Canal in southwestern England, a job that lasted six years.
The job of surveying canal routes required detailed knowledge of the rocks through which the canal was to be dug. This led Smith to examine the local rocks very carefully. While doing this, Smith observed that the fossils found in a section of sedimentary rock were always in a certain order from the bottom to the top of the section. This order of appearance could also be seen in other rock sections, even those on the other side of England. As Smith described it,
. . . each stratum contained organized fossils peculiar to itself, and might, in cases otherwise doubtful, be recognized and discriminated from others like it, but in a different part of the series, by examination of them.
This is a statement of the "principle of faunal succession." The layers of sedimentary rocks in any given location contain fossils in a definite sequence; the same sequence can be found in rocks elsewhere, and hence strata can be correlated between locations. The principle is still used today.
In 1796, Smith began to write notes and draw up local geologic maps. Smith was not the first to make geologic maps, but he was the first to use fossils as a tool for mapping rocks by their stratigraphic order.
In 1799, Smith's employment with the canal-building firm came to an end. Smith then took a series of engineering jobs in several parts of Britain, and made a number of side trips all over England and Wales. His goal was to produce a complete geologic map of England and Wales, using the principles of fossil succession. Production of the completed map began in 1812, and in 1815 the map was finally published.
Georges Cuvier
At about the same time and the Neptunists and Plutonists were discovering a long physical history of the Earth, a brilliant French zoologist named George Cuvier was proving a long biological history of the Earth.
After being appointed to the French National Academy of Arts and Sciences in 1795 at the age of 25, Cuvier quickly established a reputation as the leading expert on vertebrate anatomy in Europe.
Through the 1700's collections of bones from the alluvial deposits of Northern Europe and Siberia had been accumulating in the collections of Europe. These bones were clearly from elephants and other large animals that no longer lived in these areas.
It was not believed at the time that, prior to the appearance of man; any animal species had ever naturally gone extinct.
Cuvier's first great contribution to paleontology was a careful comparison of the bones of living elephants with those of fossil elephants bones found in northern Europe and Siberia. He was able to show without question that the fossil bones belonged to an unknown species of elephant distinct from any known alive. This was the first incontrovertible evidence for the reality of extinction.
By the early 1800's the strata of Europe were subdivided into 4 general formations that lie beneath the surficial deposits:
Excavations in the Tertiary gypsum and Secondary chalks and clays produced animals that, when reconstructed, were clearly unlike any that were known alive. Even more interesting to Cuvier was the observation that the animals became more and more unlike those of the present day the deeper (=older) the strata were that produced them.
Cuvier was impressed by the observation that the types of animals were very different from one deposit to the next, and he drew the quite reasonable conclusion that the present flora and fauna of the world was only the most recent of a successive series of worlds that stretched back into geologic time.
Cuvier eventually published his ideas on the history of life in a volume entitled Discourse on the revolutions of the surface of the globe. In his discourse, Cuvier argued that within an immensely lengthy period of earth history, long intervals of generally tranquil conditions when life flourished were punctuated by sudden major changes in physical geography that wiped out the existing organisms.
As to the origin of new species to replace the lost ones, Cuvier did not feel that enough evidence was available to usefully speculate on this. Instead, he suggested that the "most important geological problem" was to continue collecting and describing fossils so that the exact nature of each 'revolution' in the history of life could be worked out in its fullest detail.
Epilogue
So we see that by the early-1800's, it was recognized that the Earth had a long history that included both geological as well as biological revolutions that marked a series of 'ages' of the past very different from the world of today. Furthermore, it was recognized that the evidence needed to reconstruct these ages was contained in the successive layers of rock covering the surface of the Earth. The modern science of historical geology was born.
Two more stories remain to be told.
The 1800's saw the maturation of the science of stratigraphy and the creation of our modern geologic time scale. This crucial episode in the early development of the science of geology was driven by the study of fossils.
The 1800's also saw the development of the observation that life had a progressive history and the rise of theories to explain the origin of species and the relatedness of organisms through time. This culminated with the publication of Charles Darwin's theory of evolution by natural selection.