In the centre of the folds fossiliferous beds with crinoids have been found, and the black slates at the top of the series contain Myophoria and other fossils, indicating that the rocks are of Triassic age.
A structure akin to cleavage, often exemplified by slates especially when they have been somewhat contorted or gnarled, is the Ausweichungsclivage of Albert Heim.
The faces of slates have usually a slightly silky lustre due to the abundance of minute scales of mica all lying parallel and reflecting light simultaneously from their pearly basal planes.
In microscopic section the best slates show much colourless mica in small, thin, irregular scales.
Minute rods or needles of rutile are also common in slates, and well-formed cubes of pyrites are often visible on the splitting faces.