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Florence Bascom spent long days in the hills of Pennsylvania and Maryland and long hours studying thin slices of rock under a microscope. In 1896 she became the first woman hired by the US Geological Survey (USGS), and her microscope work showed that some old rocks in the eastern United States were ancient lava.
Who was Florence Bascom?
Bascom was born on 14 July 1862 in Williamstown, Massachusetts, according to a memorial by the geologist Eleanora Bliss Knopf. She graduated from the University of Wisconsin in 1882, then went to Johns Hopkins University to study petrography under George Huntington Williams. In 1893 she became the first woman to receive a Johns Hopkins PhD.
In 1895 Bryn Mawr College asked her to set up a Department of Geology for women.
What did Florence Bascom do at the US Geological Survey?
In 1896 Bascom became the first woman appointed to the USGS, as an assistant (Penn finding aid; Library of Congress). Her assignment, as Knopf's memorial puts it, was “the study of the crystalline schists in an area of one square degree” covering eastern Pennsylvania and Maryland and a corner of northwestern Delaware, later extended to the Trenton area of New Jersey.
She did this alongside her Bryn Mawr teaching. Knopf remembered the rule: in the field by seven in the morning, back around six, then an evening of drafting maps. The memorial's bibliography lists the USGS folios that followed, including:
Philadelphia district, Folio 162 (1909)
Trenton quadrangle, Folio 167 (1909)
Elkton and Wilmington quadrangles, Folio 211 (1920)
Fairfield and Gettysburg quadrangles, Folio 225 (1929)
She became vice president of the Geological Society of America in 1930 and died in Williamstown on 18 June 1945.
How did Bascom prove the rocks were ancient lava?
Her doctoral work on the old rocks of South Mountain appeared in 1893 in the Journal of Geology, and a fuller account followed as USGS Bulletin 136 in 1896. There she noted that in earlier surveys “the microscope has not been used” to work out where these rocks came from. Under the microscope, she wrote, “an increasing number of the so-called sedimentary rocks are proving to be igneous in origin.”
Why were people fooled? Bascom described round crystal clusters called spherulites that, once weathering makes them stand out, give the rock “a superficial resemblance to a conglomerate” of pebbles of remarkably uniform size. Her case was that these rocks began as glassy lava, like modern rhyolite, and later crystallised while solid, a change called devitrification.
The proof used polarised light, with thin slices viewed between crossed nicols, two polarisers set at right angles. She reported that flow, spherulitic and perlitic patterns clear in ordinary light vanish into a uniform crystal mosaic between crossed polarisers: the crystals grew after the original structures. You can see the physics of crossed polarisers in our post on testing Malus's law at 0, 45 and 90 degrees. If it is still hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
How can you read a rock the way Bascom did?
These questions, drawn from Bulletin 136, come before calling a rock sediment or lava.
Are there bands that look as if the rock once flowed? She read flow structure as a sign of moving lava.
Are the “pebbles” all the same size and perfectly round? They may be spherulites, not a conglomerate.
Are there oval holes filled with another mineral? She found gas bubbles stretched by flow and filled with quartz or epidote.
Are there curved, onion-like cracks? She called perlitic structure “a most reliable proof” of former glass, but noted its absence proves nothing.
What changes under crossed polarisers? Patterns that disappear point to crystals that grew later.
Worked example: a field note reads “purple-grey rock, small round grains all alike, a few oval holes lined with green mineral.” By clue 2 the grains could be spherulites; by clue 3 the holes could be filled gas bubbles. Two lava clues outweigh the pebbly look, so the next step is a thin section, not a sediment label.
What can students take from Florence Bascom's career?
Bascom's students went on to university posts and state and federal surveys, Knopf wrote. A 2007 paper by R. M. Clary and J. H. Wandersee concludes that it was largely through her efforts that more women were able to pursue higher education in geology and excel at it in the first half of the 20th century.
India has its own teacher who read old rocks: see how D. N. Wadia read the rocks of the Himalaya. If rocks pull you in, read how to take part in the Earth Science Olympiad in India. If the physics and chemistry of light and crystals feel shaky, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson.
Next time a rock looks ordinary, ask Bascom's question: what was it before it changed?
