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slate - teaching hand specimen of a Late Jurassic black slate from the Mother Lode belt of California

$ 7.50

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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