Two great geological periods have already come to their close; and the floor of a widely-spread ocean, to which we can affix no limits, and of whose shores or their inhabitants nothing is yet known, is occupied to the depth of many thousand feet by the remains of bygone existences. Of late, the geologist has learned from Murchison to distinguish the rocks of these two periods--the lower as those of the Cambrian, the upper as those of the Silurian group. The lower--representative of the first glimmering twilight of being--of a dawn so feeble that it may seem doubtful whether in reality the gloom had lightened--must still be regarded as a period of uncertainty. Its ripple-marked sandstones, and its half coherent accumulations of dark-colored strata, which decompose into mud, show that every one of its many plains must have formed in succession an upper surface of the bottom of the sea; but it remains for future discoverers to determine regarding the shapes of life that burrowed in its ooze, or careered through the incumbent waters. In one locality it would seem as if a few worms had crawled to the surface, and left their involved and tortuous folds doubtfully impressed on the stone. Some of them resemble miniature cables, carelessly coiled; others, furnished with what seem numerous legs, remind us of the existing Nereidina of our sandy shores--those red-blooded, many-legged worms, resembling elongated centipedes, that wriggle with such activity among the mingled mud and water, as we turn over the stones under which they had sheltered. Were creatures such as these the lords of this lower ocean? Did they enter first on the stage, in that great drama of being in which poets and philosophers, monarchs and mighty conquerors, were afterwards to mingle as actors? Does the reader remember that story in the _Arabian Nights_, in which the battle of the magicians is described? At an early stage of the combat a little worm creeps over the pavement; at its close two terrible dragons contend in an atmosphere of fire. But even the worms of the Cambrian System can scarce be regarded as established. The evidence respecting their place and their nature must still be held as involved in some such degree of doubt as attaches to the researches of the antiquary, when engaged in tracing what their remains much resemble--the involved sculpturings of some Runic obelisk, weathered by the storms of a thousand winters. There is less of doubt, however, regarding the existences of the upper group of rocks--the Silurian.