of denudation, and obliterated by processes of metamorphosis.  Beds of rock of any thickness, crammed full of organic remains, may yet, either by the percolation of water through them, or by the influence of subterranean heat, lose all trace of these remains, and present the appearance of beds of rock formed under conditions in which living forms were absent.  Such metamorphic rocks occur in formations of all ages; and, in various cases, there are very good grounds for the belief that they have contained organic remains, and that those remains have been absolutely obliterated.

I insist upon the defects of the geological record the more because those who have not attended to these matters are apt to say, “It is all very well, but when you get into a difficulty with your theory of evolution, you appeal to the incompleteness and the imperfection of the geological record;” and I want to make it perfectly clear to you that this imperfection is a great fact, which must be taken into account in all our speculations, or we shall constantly be going wrong.

[Illustration:  FIG. 2.—­TRACKS OF BRONTOZOUM.]

You see the singular series of footmarks, drawn of its natural size in the large diagram hanging up here (Fig. 2), which I owe to the kindness of my friend Professor Marsh, with whom I had the opportunity recently of visiting the precise locality in Massachusetts in which these tracks occur.  I am, therefore, able to give you my own testimony, if needed, that the diagram accurately represents what we saw.  The valley of the Connecticut is classical ground for the geologist.  It contains great beds of sandstone, covering many square miles, which have evidently formed a part of an ancient sea-shore, or, it may be, lake-shore.  For a certain period of time after their deposition, these beds have remained sufficiently soft to receive the impressions of the feet of whatever animals walked over them, and to preserve them afterwards, in exactly the same way as such impressions are at this hour preserved on the shores of the Bay of Fundy and elsewhere.  The diagram represents the track of some gigantic animal, which walked on its hind legs.  You see the series of marks made alternately by the right and by the left foot; so that, from one impression to the other of the three-toed foot on the same side, is one stride, and that stride, as we measured it, is six feet nine inches.  I leave you, therefore, to form an impression of the magnitude of the creature which, as it walked along the ancient shore, made these impressions.

Of such impressions there are untold thousands upon these sandstones.  Fifty or sixty different kinds have been discovered, and they cover vast areas.  But, up to this present time, not a bone, not a fragment, of any one of the animals which left these great footmarks has been found; in fact, the only animal remains which have been met with in all these deposits, from the time of their discovery to the present day—­though they have been carefully hunted over—­is a fragmentary skeleton of one of the smaller forms.  What has become of the bones of all these animals?  You see we are not dealing with little creatures, but with animals that make a step of six feet nine inches; and their remains must have been left somewhere.  The probability is, that they been dissolved away, and absolutely lost.