slate - teaching hand specimen of a Late Jurassic black slate from the Mother Lode belt of California
This slate is from the Mariposa Formation in the Mother Lode region of California's Sierra Madre. It originated as fine-grained marine sediments and occurs as great thicknesses of highly metamorphosed gold-bearing slates in the Sierra Nevada foothills. The U.S. Geological Survey maps this as Late Jurassic in age.
There is a sequence of increasing compaction from sedimentary mudstone and shale to metamorphic slate. Shales are somewhat compressed and can resemble slates, as they both originated as fine-grained muddy sediments. A shale will often smell strongly muddy when licked. A slate, metamorphosed from shale by greater compaction and/or heating, will either not smell muddy, or will only smell slightly muddy when licked, a good field identification. This slate passes the sniff test, with a faint or no muddy smell when wetted. Slate often can be parted in large smooth-surfaced sheets. Large sheets of black slate were used as the chalkboard in schools is the origin of the term blackboard.
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