Pennsylvanian time

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There were so many such cycles at close intervals in Pennsylvanian time that Pennsylvanian rock sequences are often striped like regimental ties-the signature of glaciers half the world away. They existed three hundred million years ago, and glacial patterns of that kind have not been repeated until now, when the measure of our own brief visit to the earth is being recorded as a paper-thin stripe in time. On both sides of the interstate, above the silhouettes of co-working space haarlem screening trees, we saw the tops of draglines-the necks and heads of industrial giraffes. They and predecessor machines had been working for fifty years, altering the topography, stripping the coal beds of Pennsylvania-in all, a mineral deposit worth a great deal more than the diamond mines of Kimberley and the goldfields of the Klondike. Coal was in the roadcuts now and would continue to be for many tens of miles-in layers that were not the dull deep gray of the Allegheny shale but truly black and shining. Layered light and dark, the roadcuts looked like Hungarian tortes. Reading up from the bottom, there was sandstone, siltstone, shale, coal, sandstone, siltstone, shale, coal. We would see limestones farther on, capping the coal where sea had covered the swamps. The present sequence was built behind a coastline-as is happening now, for example, in the bayous of the Mississippi Delta-by rivers meandering to and fro, covering with sand the matted vegetation. “These roadcuts are a textbook on the making of coal,” Anita said. Buried and co-working space groningen compressed, vegetal debris first becomes peat-a melange of spores, seed coats, wood, bark, leaves, and roots which looks like chewing tobacco and burns about as well. Peat bears much the same relation to coal that snow does to glacier ice. As snow is ever more buried and compacter!, it recrystallizes and becomes ice-on the average ten times as dense as the original snow. As peat is buried, compacted, subjected to geothermal heat, it gradually gives up much of its oxygen, hydrogen, and nitrogen, and concentrates its content of carbon. The American Geological Institute’s Glossary of Geology defines coal as “a readily combustible rock.”

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