Lyell and the Principles of Geology

Transfer of ideas

• Analogies to human history are obvious from first page
• Past errors of geology are due to
  • Lack of appreciation of the time scale; Lack of imagination
  • Viewpoint as terrestrial beings
  • Confusing geology with cosmology (origins)

Processes

• Physical
  • Aqueous
    • Erosion and deposition at surface
    • Wide range of modern processes described
  • Igneous and Earthquakes
    • Examples described
    • May build up or decay the landscape
    • Source of magmas not really specified (not accept internal heat)
  • Argues that these modern processes (acting at the current rates) can form all the features of the present physical world.
  • Argues for a long-term balance of uplift and subsidence
    • No net change in area of land versus sea
    • Position of land may shift over time
• Biological
  • Species are natural units
  • Species originate at a single place that they are suited to live in
  • Origination is due to an unknown natural mechanism
  • Migrate into appropriate environments
  • Go extinct when environment changes
  • Species distribution in constant flux but relatively uniform in time
  • Result is a gradual turnover through time
• Interactions of Physical and Biological
  • Physical processes will result in a gradual changes of paleogeography
  • Changes will drive environmental changes
  • Leading to extinctions of fauna and flora
  • Leads to a constant flux in the fauna and flora

Climate Model

• Lyell’s answer to directional history of life
  • Builds on his view of processes and their interactions
• Faunal Progression
  • Sequential faunal appearances of invertebrates, fish, amphibians/reptiles, mammals, “cold climate” mammals (mammoths), humans
  • Parallel development in plants: ferns, gymnosperms, angiosperms
  • All interpreted as due to progressive cooling of earth through time
• Secondary to Tertiary = the cooling half of a grand climate cycle
  • Driven by shifting positions of land through time
  • Uses European record as evidence
    • Secondary: Marine and swamps = warm
    • Tertiary: Mixed terrestrial and scattered basins = cooler
    • Modern: terrestrial = coldest

Historical Geology

• Outline by topic
  • General principles (ch 1-4)
  • Tertiary rocks (5-22)
  • Secondary rocks (ch 23)
  • Mountain building (ch 24)
  • Primary rocks (ch 25-26)
• Diagrams and Tables summarize his point
  • Constant turnover of species suggest long time scale and gaps
  • “Primary” rocks are actually a mixture of igneous intrusions and metamorphic (his new term!) rocks
  • Igneous rocks that cross-cut younger strata indicate that they form throughout earth history
  • Same types of environments through the Tertiary and Secondary deposits => NO net change

Uniformity (more or this soon!)

• Natural processes
• Actualism (modern processes)
• Gradualism
• Steady-State