The same course of argument applies to every one of the formations, to some of them of vast thickness, even more conclusively, where we find the same species persistent throughout the group, and the same genus often extending over two or three entire formations, embracing periods of geological time of as many millions of years. Thus the _Leptœna lata_ of the Silurian age lives on to the close of the Carboniferous; the _trilobite_, earliest of living creatures, has its representatives still in our modern seas; the mail-clad _holoptychius_ existed through the whole of the Devonian and Carboniferous eras; and equally remarkable is the fact that the _Onchus Marchisoni_, the oldest fish yet detected in the rocks of the earth, is a creature more allied to the existing genus _Spinax_ (the dog-fish) than to any other family of relics inclosed in all the intermediate ascending series of deposits. Among the infusoria it is ascertained that there’ are two kinds of living _Gallionellæ_ identical with the fossil species in the Richmond clays of Virginia; while again, in geological botany, we have all the types of the coal formation still flourishing with the sane gigantic forms in the continents and islands washed by the Pacific. 3. THE SUPERFICIAL ACCUMULATIONS. The argument of the geologists, for their indefinite periods of time, proceeds mainly upon the assumption that the present and the past operations of the laws of nature are nearly uniform; or, in other words, that the existing rate of increment of detrital and alluvial matter, in seas, deltas, and rivers, is to be taken as the standard throughout the various geological epochs. Tried by the test of the superficial accumulations, the subject is brought within a manageable compass, the definite is substituted for the indefinite, and the scale of accumulative power in the ancient will be in the ratio of its erosive and transporting agency in the modern epoch. The products of volcanoes also fall to be considered in estimating the effects of causes now in operation.