In all of them there is at least one mineral which crystallizes in platy forms (e.g.
The platy minerals have also a perfect cleavage parallel to their flat surfaces, while the fibrous species often have two or more cleavages following their long axes; hence a schistose rock may split not only by separation of the mineral plates from one another but also by cleavage of the parallel minerals through their substance.
The difference between schists and gneisses is mainly that the latter have less highly developed foliation; they also, as a rule, are more coarse grained, and contain far more quartz and felspar, two minerals which rarely assume platy or acicular forms, and hence do not lead to the production of a fissile character in the rocks in which they are important constituents.
Micas and other platy minerals (such as chlorite), which naturally grow most rapidly on their edges, would show this tendency best, and such minerals usually form a large part of the best slates; but even Sketch (by Du Noyer) of a block of variegated slate from Devil's Glen, Co.
These are overlaid conformably by a thick series of platy limestones, known as the Viglas limestone, which appears to represent the rest of the Jurassic system and also the lower part of the Cretaceous.