slate-clay

Penas Negras rests on a slate-clay, mixed with quartzose sand; but there is no proof of the muriatiferous clay of the salt-works being of more ancient formation than this slate-clay, or of its alternating with banks of sandstone.

Travels to the Equinoctial Regions of America 1851

Negras rests on a slate-clay, mixed with quartzose sand; but there is no proof of the muriatiferous clay of the salt-works being of more ancient formation than this slate-clay, or of its alternating with banks of sandstone.

Personal Narrative of Travels to the Equinoctial Regions of America, During the Year 1799-1804 — Volume 1 Alexander von Humboldt 1814