Page:A Treatise on Geology, volume 1.djvu/141

 CHAP. VI. "which was established upon the globe." But there is yet another aspect of this question. Sir R. Murchison, when he proposed the Silurian system, was perfectly aware of its divisible character, and did in fact divide it into upper and lower. To each of these divisions belongs a large suite of fossils, and they are so distributed that only a small percentage passes from the one to the other. If upon this basis we were to cut the original Silurian series into two systems, the Lower Silurians would attach to themselves all the strata above the hypozoic series, and this is really the Cambrian system of Sedgwick. But this will hardly be done without hesitation, and without a more full discussion of evidence from all quarters of the palæozoic strata, than has yet been attempted. In the Malvern district, though upon the whole the fossils of the upper and lower Silurians are very distinct, limestones which occur in the lower group are full of fossils which are generally viewed as belonging to the upper, but they occur below sandstones which contain the fossils of the lower series.

Perhaps no conclusion now to be adopted can be entirely satisfactory. As the matter stands at present we prefer to keep the Silurian system within its original limits, and to treat separately the strata between it and the hypozoic rocks, as a series in which the rarity of life contrasts strongly with the rich variety of organization which fills the true Silurians. It is to professor Sedgwick we are mainly indebted for our knowledge of them. In his nomenclature they are ranked as lower Cambrian.

Composition. The type of these rocks is upon the whole eminently argillaceous, as that of the older systems is arenaceous: but between these two terms the difference is not always very clear. Some proportion of alumina must, indeed, be present in argillaceous rocks, but it is seldom absent from arenaceous compounds: such a substance as felspar, reduced to fine particles in water, might make a good substitute for clay; if left in a state