Metamorphosed sedimentary rocks are widely distributed in the schistose series, but they are distinctly subordinate to the meta-ignecius rocks, and they are so highly metamorphic that stratigraphic methods are not usually applicable to them.
Observing for himself (1794-1800) the stratigraphic value of fossils, he began to distinguish the great Mesozoic formations of England (1801).
Locally both the sedimentary and igneous parts of the group have been highly metamorphosed; but as a rule the alteration of the sedimentary portions has not gone so far that stratigraphic methods are inapplicable to them, though in some places detailed study is necessary to make out their structure.
As a result of this emergence the stratigraphic break between the Ordovician and the Silurian is one of the greatest in the whole Palaeozoic group.
The subject will be treated in its biological aspects, because the relations of palaeontology to historical and stratigraphic geology are more appropriately considered under the article Geology.
Not only is it extensive in area, but the stratigraphic break is very great, as shown by (I) the excess of metamorphism of the lower group as compared with the upper, and (2) the amount of erosion suffered by the older group before the deposition of the younger.