shale - hand specimen of a hard green Lower Cambrian shale
This shale is part of the Lower Cambrian Poleta Formation, from the White Mountains of California. These specimens are a fairly hard shale, so they will not crumble during student examination as will more fissile shales. Fissile refers to the behavior of shales that weather out as a pile of loose platy flakes. This shale passes the lick test - it smells slightly muddy - see below. I would ask students for the reason for their identification. If they called this a shale because it smells muddy when wet, or if they called it a slate because it does not smell muddy, I would accept either answer. This particular shale exhibits platy parting - it looks like a shale in the outcrop, but its hardness is variable.
Students should know the basic sedimentary rocks in order of increasing grain size, from mudstone and shale, siltstone, sandstone to conglomerate and coquina. Shales differ from mudstones in that they are more compressed, form flat plates, and are less likely to disappear in a poof of dust when dropped by a student. This shale is fairly hard and should stand up to student examinations.
Some shales closely resemble slate, metamorphosed from shale. The shale in the Poleta Formation has been slightly metamorphosed, but not enough to destroy fossils. Disarticulated trilobites are common in some locations, and the heads frequently are distorted by metamorphism. These specimens do not contain fossils. To distinguish a shale from a slate, the test is to lick it. The shale will smell muddy. The slate won't.
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