best exhibited. The farther west we go in Wales, the less obvious is the division of the formations; the greatest degree of distinctness is observable in the eastern districts; we may by selecting examples find that the characters of the whole series graduate from one group to another. There is so great and real an analogy between the Llandeilo, Caradoc, Wenlock, and Ludlow formations, as to permit our viewing them all as one varied series of deposits effected by one general system of mechanical, chemical, and vital agencies. Sir R. Murchison, by his long researches on the border of Wales, added to the magnificent series of English strata and palaeozoic life this one of the most important of their terms. He thus advanced the principles of W. Smith to a new and most fruitful conquest, and established results which have and must retain a powerful and beneficial influence on future geological discovery.
Organic Remains.—In his great work, "the Silurian System," (1836) Murchison records 351 species of fossils[1] which had come under his notice, and describes in respect of each its geological and geographical ranges as then determined. A similar investigation made by the author of this work on the materials collected by the Geological Survey of Great Britain (1848), gave 353 species for the districts between the Severn and St. Bride's Bay. The following table represents the geographical distribution of these 353 species in thirteen districts.
The two most prolific districts in this table are those of Malvern and Llandeilo, which are the two containing the most complete series of Silurian strata. The two least productive are those of Cardigan and Caermarthen, in which only the lowest group appear. The eastern districts are more prolific than the western, because they exhibit a greater variety of mineral character in the deposits.
- ↑ Exclusive of fishes which occur, indeed, in the upper Silurians, but not so frequently or regularly as to be of use in arguments depending on numerical data. They are all of small size, incompletely known, probably confined to the placoid group, and perhaps to the upper Silurian strata.
