Continuing to trace the history of this ancient period, we reach what is called among geologists the Old Red Sandstone age. The corals, and the shell-fish, and the crustacea of the former period have passed away, and in their place we find _fishes_; thus presenting to us the earliest trace of the highest order of the animal kingdom--vertebrata. The plants in this system are few, and it would seem as if the condition of the world was ill-adapted for their growth. Another peculiar characteristic of this era is the state of calm repose in which the ocean appears to have remained; in many rocks the _ripple mark_ left by the tide on the shores of the ancient seas is clearly visible; nevertheless considerable volcanic action must have taken place, if we are to believe geologists, who find themselves unable to account otherwise for the preponderance of mineral matter which seems to have been held in solution by the waters. We now pass on to the Carboniferous period, and a marked change at once strikes us as having taken place. In the previous era few plants appear to have existed; now they flourished with unrivaled luxuriance. Ferns, cacti, gigantic equisetums, and many plants of which there are no existing types, grew, and lived, and died in vast impenetrable forests; while the bulrush and the cane, or genera nearly allied to them, occupied the swamps and lowlands. This is the period when the great coal beds and strata of ironstone were deposited, which supply us with fuel for our fires, and materials for our machinery. The interminable forests that grew and died in the lapse of centuries were gradually borne down by the rivers and torrents to the ocean, at whose bottom they ultimately found a resting place. A considerable portion of the land also seems to have been slowly submerged, as in some cases fossil trees and plants are found in an upright position, as they originally grew.