These eras may have been some longer, some shorter, but each had a duration which, when tried by human standards, must be regarded as immensely protracted. The cycles of astronomy are very vast, yet I have often thought that the cycles of geology, though probably of much less duration, impress us more forcibly with the antiquity of our planet. The astronomer tells us of light that has taken two millions of years to reach our earth, and of nebulæ that are millions upon millions of miles distant, but these numbers are so vast that we cannot bring ourselves to realize them. We _know_ that there is a great difference between two millions and ten millions, but we cannot fully _appreciate_ it, and so the periods of the astronomer, beyond a certain point, cease adequately to impress us. So long as they can be easily contrasted with our own standards of comparison, they have their full force; but after that, every additional million, or ten millions, or ten hundred millions, produces only a confused and bewildered sense of immensity, and the comparative amount of each addition fails to be realized. Will my reader forgive a homely illustration:--Some years ago, I stood at the pier-head of one of our smaller sea-port towns, and watched the sun as it sullenly sank behind the outline of the opposite hills. The breadth of the channel, in the direction of sunset, was several miles, but in the flush of evening one fancied he could almost have thrown a stone across. The water lay unruffled by a ripple, and reflected all the thousand varying tints that lighted up the sky. The harbour, that had been a busy scene all evening, began to grow less noisy, as one by one the herring-boats pushed out to sea. I found it not a little interesting to mark, as the boats gained the open firth, how the opposite coast-line gradually seemed to recede. The farther the dark sails withdrew, the more remote did the adjacent shores appear, until, as the last tinge of glory faded from the clouds, and a cold grey tint settled down over the landscape, the hills lay deep in shade and stretched away in the twilight as a dark and distant land from whose valleys there rose troops of stars. The coast-line, as seen in early evening, reminded me of the periods of the astronomer; as seen in early night, it reminded me of the periods of the geologist. We fail to appreciate the real duration of astronomical cycles, because they are presented to us each as one vast period. They are not subdivided into intervals, and contain no succession of events, by means of which, as by milestones, we might estimate their extent; and so their unvaried continuity tends to diminish the impression of their vastness, just as the firth, without any islet or vessel on its surface, seemed greatly narrower than it really was. For it is with time as it is with space--the eye cannot abstractly estimate distance, nor can the mind estimate duration. In either case, the process must be conducted by a