Sedgwick attacked the problem in the Snowdon district, where the rocks are highly altered and displaced and where fossils are comparatively difficult to obtain; Murchison, on the other hand, began to work at the upper end of the series where the stratigraphy is simple and the fossils are abundant.
The rocks, for convenience classed as pre-Cambrian, occur as several unconformable groups, chiefly developed in the south where alone their stratigraphy has been determined.
The rocks of Secondary and Tertiary ages have been profoundly affected by the Alpine movements, and are thrown into a series of complex folds, so that in numerous instances their stratigraphy is imperfectly understood.
We shall pass over here the labours of Adam Sedgwick (1785-1873) and Sir Roderick Murchison (1792-1871) in the Palaeozoic of England, which because of their close relation to stratigraphy more properly concern geology; but must mention the grand contributions of Joachim Barrande (1799-1883), published in his Systeme silurien du centre de la Boheme, the first volume of which appeared in 1852.
His famous " law of correlation," which by its apparent brilliancy added enormously to his prestige, is not supported by modern philosophical anatomy, and his services to stratigraphy were diminished by his generalizations as to a succession of sudden extinctions and renovations of life.
The term is a relic of that early period in the history of stratigraphy when each group of strata was supposed to be distinguished by some peculiar lithological character.
The history of invertebrate palaeontology during the second period is more closely connected with the rise of historic geology and stratigraphy, especially with the settlement of the great and minor time divisions of the earth's history.