The fine mud which now appears as beds of slate was the decay of feldspar and hornblende in the same granite.
"Earth and Sky Every Child Should Know" by Julia Ellen Rogers
Their geological formation is granite, with gneiss and mica slate, with clay slate, hornblende and primitive limestone overlying.
"Encyclopaedia Britannica, 11th Edition, Volume 11, Slice 8" by Various
The Serpentine rock streaked with hornblende, felspar, slate and green-stone, shows changing colours like a pigeon's breast.
"Cornwall" by G. E. Mitton