The development of foliation in shaly rocks is undoubtedly closely akin to the production of cleavage in slates.
In thickness, but in this case layers of shaly coal are included.
The sea grew shallow, the deposits became conglomeratic and shaly, volcanic eruptions began, and the present folds of the Apennines were initiated.
A black and often shaly type called " calp " contains much clay derived from the same land-surface.
As the impurities increase in amount the clay rocks pass gradually into argillaceous sands and sandstones, argillaceous limestones and dolomites, shaly coals and clay ironstones.