Geognosy

Geognosy as a “Structural Science” (focus of Rudwick reading)

• Based on “known” facts about the earth
  • Basic constituents
    • Minerals: defined by chemistry, identified by external characteristics (Werner)
    • Rock types: made of one or more minerals
    • Most rocks are of one of two textures:
      • Crystalline: due to precipitation from solution
      • Particulate (clastic): particles eroded and deposited
  • Fluids and minerals/rocks
    • Early earth was hot; cooling ever since
    • Even silicates were “fluid” when heated
    • Natural waters contain dissolved solids which can precipitate
    • Sea level is falling (Baltic)
  • Structure of Mountains: Archaic Time Scale
    • Divided into four basic groupings
    • Primary, Secondary, Alluvial/Tertiary, Volcanic (or similar scheme)
      • Each with its own specific geometry, altitude
      • Age relations due to superposition
      • Each of these consist of a number of Gebriges (new term!)
        • Specific collection of rock types
        • Linked together into the major structural entities
• Origins of geognosy
  • Mining tradition
  • Stress on three dimensions
• Common framework of natural history studies of the earth

Geognosy as a “Theory of the Earth” (focus of Laudan reading)

• Integrate positions, relative ages, compositions, implied processes of origin
• Use geobriges as key factors to tracing changing conditions through time
• Basic “story”
  • Original earth hot, molten; cooled through time
  • Primary
    • Oldest unit and also highest elevation, crystalline rocks formed under high sea levels
    • Precipitated from hot solutions very different from present ocean
    • Granite, gneiss, marble, quartzite
  • Secondary
    • Next unit; deposited on flanks of primary as sea level falls with some oscillations
    • Mixture of gebriges
      • Precipitated: gypsum, limestone, basalt (debatable – next class); implies composition of oceans more like present
      • Debris eroded off Primary: sandstone, shale, breccia
    • Fossils appear, along with coals
  • Tertiary/Alluvial
    • Lower elevations, flat-lying strata
    • Sea level fall to near present levels
    • Gebriges
      • Almost all clastic debris: sandstone, shale, breccia
      • Minor amounts of (precipitated) limestone and gypsum
      • Modern looking fossils
  • Volcanoes
    • Attributed to the underground burning of coals (positioned in the Secondary)
    • Inferred to be very recent
• Basic history was widely accepted BUT left room for some variation in interpretation (see next class).