Crossword Genius

Fissile rock (5)

Ross

I believe the answer is:

shale

'fissile rock' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for shale that I've seen before include "Clayey rock" , "Easily split layered rock" , "Stratified rock" , "Oil-bearing rock" , "Stone that splits easily into fine pieces" .)

I've seen this clue in the LA Times, The Wall Street Journal and the Penny Press.
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