Andalusite-bearing slate

Var. chiastolite. Andalusite is stable at low pressures and temperatures.

Slate

Slate shingles cover the Museum's central dome and roof. Slate makes good shingles because it splits easily into thin, durable sheets of solid rock. Why? It contains platelike mica minerals arranged in narrowly spaced layers.

Slate with deformed trilobite fossil

Deformation related to faulting flattened the black fractured rock and its fossil at an angle to the original layers. You can see the result: a long, asymmetric trilobite.

Slate with Chevron folds

Deformed while cooler and less ductile, this slate buckled into more angular folds.

Slate with undeformed trilobite fossil

Flowing rocks can be stretched into elongated forms. Compare this specimen and the other piece with the same number. Once, both of these trilobite fossils looked alike.