Chapter 35
Geology: the structure and history of the earth; rocks, strata, fossils, glaciers, volcanoes and the forces that shape them.
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Geology, as the Harvard Classics take it up, is the study of the earth’s structure and history: its rocks and strata, the fossils buried in them, the glaciers and volcanoes that work on its surface, and the forces that raise mountains and wear them down. It is a late science, and in this collection its systematic voice belongs mostly to the nineteenth century, to Lyell, Darwin, Geikie, Helmholtz and Kelvin. Yet the questions it answers are very old. Scripture, myth and poetry asked how land was parted from sea, why mountains shake and smoke, and what the shells and bones in the rock might mean. The idea is great for these books because it changed how men measured time. Once the earth was read as a document written slowly over immense ages, the history of life, the scale of human history and even the method of reasoning about the past had to be rethought. Emerson saw this clearly. Geology, he says, replaced the Mosaic chronology with nature’s patient periods, in which rock is formed and broken, lichen makes soil, and the trilobite, the quadruped and finally man appear in turn1.
The central methodological quarrel, as Lyell tells it, lay between catastrophe and uniformity. His history of the science describes a shift. Earlier geologists believed that distinct causes had acted in a primeval world. The newer view held that the laws now in operation explain all former changes of the earth’s surface2. The older error, he argues, sprang from a mistaken idea of the earth’s age. If one crowds the building of volcanic cones, the flow of lavas, earthquakes and the growth of deltas into a short span, the record will seem to show violent revolutions. The Chilean uplifts, by contrast, show that mountain chains could be built gradually3,4. He rejects the Woodwardian flood hypothesis5. He tells how the basalts of the Vicentin came to be recognized as ancient submarine lavas by comparison with active volcanoes, which overturned the theory that such rocks were precipitated from a chaotic fluid6. His conclusion is that geological phenomena should be explained by ordinary causes operating over vast time7,8.
Darwin took over this doctrine and made it an analogy for natural selection. Slow agencies still at work excavate valleys and cliffs, and this displaces the notion of a single diluvial wave9. He compares resistance to his theory with the resistance once offered to Lyell10. Geikie closes his survey in the same spirit: rain, rivers, glaciers, earthquakes and volcanoes are shaping the land today11.
The older writers had held the catastrophic view without argument. Bacon treats deluges and earthquakes as the great destroyers of past ages. He explains the fall of Atlantis by a long, shallow flood and supports this by pointing to the greater rivers and higher mountains of America12,13. Rousseau has islands torn from continents by inundations, earthquakes and “revolutions of the globe”14. Virgil’s Helenus says an earthquake split Sicily from Italy and the sea rushed into the rift15. Milton has the Flood tear the mount of Paradise from its place and leave it an island16.
Uniformity required an immensity of time, and the moderns tried to gauge it. Darwin measures it by denudation and by the thickness of sediment. Thick formations, he notes, imply equal erosion elsewhere in the crust17,18. He totals the British strata at more than seventy-two thousand feet, notes the gaps between formations, and cites Croll’s rate of denudation19. He also observes that flint tools in superficial deposits place man’s own existence at a remote period20. Geikie calculates that rivers could level Europe in a few million years. He estimates the bulk of Silurian rock eroded from a vanished northern land21,22. Lyell adds the Sicilian Tertiary limestone, raised thousands of feet and then cut into valleys23. Whitman wants a philosophy wide enough to comprehend the “patient upheaving of strata”24.
Lyell also asks why observers misjudge the past at all. Living on land, they see rocks decompose and rivers carry detritus. They do not see new strata forming under water or igneous rocks cooling far below25,26. He imagines a subterranean being who would wrongly take the fossil-bearing rocks to be the oldest27. He offers the figure of a sleeper who wakes to find everything changed and supposes a revolution has occurred28. The remedy, he says, is to read ancient deposits by present subaqueous and subterranean processes29. Darwin gives concrete warnings of the same kind. Cattle killed by drought and buried by floods might lead a geologist to infer a deluge30. Tropical-looking shells might suggest a tropical climate that never existed31.
As a branch of learning, geology meets both praise and doubt. Milton’s curriculum reaches minerals only as a stage in natural philosophy32. Goethe urges artists to know the nature of stones33. Emerson notes that English boys know all that Hutton knew of strata34. Elsewhere he complains that the geologist lays bare the strata but cannot say what granite or marl does to the people who live on it35. Geikie joins geography and geology as two eyes upon the earth’s surface36.
The question of the earth’s origin first appears in poetry. The Psalmist describes foundations laid, the deep covering the mountains, and the waters retreating as the mountains rose and the valleys sank37. Milton’s waters drain off and carve channels in the ooze38. Socrates’ myth in the Phaedo treats the corroded stones, sand and mud of our region as the sediment of a nobler world39. Dante makes the land flee Satan’s fall and the earth veil itself with sea40.
The modern question about the interior is sharper. During the Chilean earthquake Darwin felt the ground move like a thin crust over a fluid41. Kelvin, reasoning from the tides, rejects the geologists’ picture of a fifty-mile shell over molten lava. Tide-generating forces, he argues, require an earth about as rigid as steel. Underground heat does not justify assuming that the interior was fluid in the past42,43,44,45. Here physics corrected geology, and the passages leave the two sciences in some tension.
On the materials of the earth, Geikie gives the fullest classification. Shales, sandstones and limestones are made of worn fragments laid down under the sea, and their marine fossils show that the land began on the sea floor46. Ripple-marks and sun-cracks show shallow water47. The deep-sea oozes dredged by the Challenger accumulate so slowly that they differ from the strata of the continents. From this he infers that ocean basins and continents are both ancient48,49. Igneous rocks are lavas or intrusions50. Metamorphic rocks are sediments recrystallized under pressure, which grade into granite in the cores of mountains51. Darwin, in the field, finds the Brazilian coast granitic, crystallized under heat and pressure52. He calls granite the deepest known layer of the crust and a classic subject of dispute53. He notes how vast denudation has laid such rocks bare54.
Mining and ores tie the science to an old human craft. Job describes shafts sunk to veins of silver and gold and mountains overturned at the roots55. Marcus Aurelius calls gold and silver the earth’s sediments56. Raleigh distinguishes marcasite from true gold-bearing white spar57,58. Harrison catalogues England’s native minerals59. Explanation of how ores form comes later. Faraday ascribes the Ballarat nugget and mineral veins to slow chemical and electric action over ages60. Darwin compares nature to a gold mill: mountains wear to mud and the heavy, indestructible gold is left behind as residue61. Darwin also studies lesser formations. He describes lightning-fused tubes of sand62, salt and gypsum crystallizing in brine lakes63, and nitrate beds left in a basin that was once a lake or an arm of the sea64.
The strata are the heart of the subject because they make the crust a history. Geikie states the principles. Superposition gives order. Fossils identify formations, with older strata holding more extinct species. Fossils can even reveal sequences overturned in mountains, though absolute time cannot yet be measured65,66. Lyell shows Tertiary deposits approaching the living fauna by degrees, without convulsion67,68. Darwin stresses how the Chalk is recognized worldwide by its fossils69,70. Geikie then narrates Europe’s growth as a connected story. It runs from the gneiss foundation through the coal swamps, the Triassic salt lakes and the Alpine folding to the volcanic outpourings of later ages71,72,73,74,75.
The record, however, is broken. Here Lyell and Darwin argue most closely together. Lyell shows that deposition is local and shifting. Fossils are preserved only under certain conditions. Thick strata build up where the sea floor sinks, while elevation leaves thin deposits that are soon dispersed76,77,78. Darwin makes this the main defence of his theory. The crust is a vast museum, but poor and haphazard. Its fossiliferous formations accumulate during subsidence and are destroyed by the action of the coast during uplift, which leaves blank intervals79,80,81,82. Upright fossil trees at many levels in Nova Scotia reveal hidden gaps of time83. The apparently sudden appearance of the Cambrian fauna implies vast unrecorded ages before it. For those ages he weighs the estimates of Thomson and Croll and the possibility that older continents lie buried beneath the oceans84,85,86. Negative evidence from the fossils is unreliable, he warns, since new finds keep pushing first appearances further back87,88.
That fossils are the remains of once-living creatures had itself been a conquest. Lyell recounts how the Subapennine shells were shown to be organic, refuting theories of a “plastic power” in stone89. He cites Falloppio’s view of elephant tusks as concretions90. Darwin dismisses the cosmogonists who thought fossil shells were created in the rock91. His own finds in the Pampas included Toxodon, Mastodon, giant armadillos and an extinct native horse. They lay in a deposit he took to be formed under water, against Bravard’s sub-aerial view92,93,94. From this geology he concluded that change was slow and that no global catastrophe had swept these animals away95. Writers outside science borrowed the figure. Emerson calls language “fossil poetry”96. Taine reconstructs a man from a document as a naturalist reads an animal from a fossil shell97. Freeman holds up the geologist’s certainty about the order of strata as a standard of evidence that history cannot match98.
Erosion is the slow chisel of the uniformitarian world. Job had already seen waters wear stones and mountains crumble99. Shakespeare saw the hungry ocean gaining on the shore100. Darwin describes erosion by waves, rain, frost and rivers, and the Wealden escarpments carved by differential denudation101. He hears mountain torrents grinding stones and reflects that such slow, repeated causes can account for thousands of feet of sediment102. Geikie adds that the waste of one land supplies the material for future lands103. On valleys, Darwin rejects both floods and rivers. He judges the Santa Cruz valley to be the work of an ancient sea strait104,105. He weighs subsidence and marine action for the great valleys of New South Wales106,107.
Elevation and subsidence complete the cycle. Lyell cites Sweden rising and Greenland sinking108. Emerson grasps the geologist’s view that the land is in perpetual flux while the sea keeps its level109. Darwin’s Beagle narrative is full of proof. Shells of living species lie hundreds of feet above the Chilean coast110,111. The earthquake at Concepción raised the land permanently112. Patagonia rose in steps, with pauses between113. Fossil shells in the Andes record fourteen thousand feet of uplift after earlier subsidence114. A petrified forest tells of land raised, sunk, buried and lifted again115.
On the building of mountains, the authors part. Darwin, from the Andes, thinks uplift and volcanic action are one force, and that ranges are built by repeated injection of plutonic rock116,117. Geikie explains folding by the contraction of a cooling earth, the crust wrinkling like a shrivelled apple. He even suggests that continents result more from the uneven subsidence of the ocean floors than from upheaval118,119. Darwin’s coral theory shows his method best. Reef corals cannot build below twenty or thirty fathoms, so atolls must rest on subsiding foundations. Fringing reefs become barrier reefs and then atolls as the land sinks, and a map of reef types becomes a map of the crust’s movements120,121,122,123.
Volcanoes and earthquakes have the oldest witnesses. Pliny describes the falling pumice, the retreating sea, the darkness and the continuing shocks of Vesuvius124,125. Darwin describes the Valdivia earthquake and its fissures, shivered slate and stones cast up from deep water126,127. He links simultaneous eruptions along the Andes to a common subterranean source128,129. He notes that active volcanoes cluster in rising areas and are absent from subsiding ones130. He judges oceanic islands to be volcanic or coral, not fragments of sunken continents131,132.
The mythic accounts are another way of explaining the same events. Virgil buries Enceladus under Etna to account for its quaking133. Dante ascribes the ruined cliffs of Hell to the earthquake at Christ’s death134, and Statius explains Purgatory’s trembling by wind pent in the earth’s hollows135. The editor of Aeschylus traces the Titan myths to the volcanic character of Asia Minor136. Pope traces superstition to fear of the rending earth137.
Ice was the last agent to be recognized. Helmholtz explains the snow line, the compaction of firn into ice, and glaciers flowing like rivers at measured speeds. He shows crevasses torn by differences in velocity and moraines formed from fallen rock138,139,140,141,142. He then reasons from polished rocks and erratic blocks to the former vast extent of Alpine ice, and he identifies loess as the dust of ancient glaciers143,144. Darwin ascribes erratic boulders to icebergs145,146. He surveys glacial traces worldwide and records Croll’s astronomical theory of recurring ice ages147,148. Geikie spreads an ice sheet over Europe149. The poets saw the same ice without explaining it. Coleridge asks who made the glaciers150, and Byron hears Mont Blanc’s glacier moving day by day151.
Several questions remain open within these pages. Kelvin’s rigid earth sits uneasily with the fluid interior the geologists assumed. Contraction and plutonic injection compete as explanations of mountains. Absolute time remains, in Geikie’s word, unmeasured66. Darwin admits that the record may never reach the earliest forms of life152. What united these writers was a conviction that the past is to be read through causes now at work, a conviction the older poets and chroniclers did not share.
Introductory essay written by Claude Opus 5.5 from the outline and the notes on every passage below; quotations are checked against this edition.
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