Common clay-slate is a type of the structure.
"Geology"
James Geikie
So as we were making the journey on horse back, we crossed Applegate about twenty miles above the bridge and pursued our journey along and over the foothills on the left bank of the river, intending to stop at a hotel on slate Creek on the left bank of the Applegate, and on the north bank of said creek about two miles from said hotel.
"Memoirs of Orange Jacobs"
Orange Jacobs
As the day was fast waining, we left them still fighting, determining that we would go to slate creek-cross it-get some rifles from our host, and then return; but when we came to slate creek, we found it a raging torrent-overflowing its banks, and spreading out over its narrow valley.
"Memoirs of Orange Jacobs"
Orange Jacobs