Page:A Treatise on Geology, volume 1.djvu/158

 142 limestones. The series of life appears at first small or null (the lower strata possibly belong to a period older than the true Silurians); then swells to a full development in Upper Caradoc; sinks to a small amount in Wenlock shale; expands to its fullest extent in Wenlock limestone; contracts again; amplifies again to the Upper Ludlow, and dies away in the Downton sandstone, which unites the Silurian rocks to the superjacent old red sandstones. Viewed on a larger scale it appears that only a small portion of the fossils of the Silurian system has been found both in the upper and lower divisions— between which undoubtedly, as sir R. Murchison first remarked, a rather strong line is traceable, especially in the eastern regions, where the upper division is calcareo-argillaceous, and the lower one arenaceous. The difference is less obvious in the extreme west, where the limestones nearly vanish. These limestones mark intervals of rest, in the generally descending movement to which the bed of the Silurian sea was subject, a movement which was continued through later periods, so as to cause the accumulation of many thousands of feet of other sediments brought from other points of the earth's surface by other currents, charged with other forms of marine life.

To obtain these 353 species 313 localities were searched diligently. The specimens obtained at almost every locality, even in a limited district, offer something peculiar to and characteristic of place; still greater is the difference between different districts; only $$\scriptstyle \frac {96}{334} = 29$$ per cent being found in each of the two neighbouring districts of Abberley and Woolhope, which contain the same strata equally exposed. A few species have very wide geographical ranges: such are:—