STANDING by the new lighthouse at Beachy Head, in front of the grand cliff section there exposed, even the casual and ungeological observer must be struck by the curious black lines in the chalk. These are the well- known flints of the Upper Chalk. We notice how they occur here in more or less parallel lines, following the stratification of the chalk. Also that they are some three or four feet apart. They present a geological problem of considerable difficulty. So far as the writer is aware no theory which goes into precise details as to the actual method of the formation of these lines of flints has yet been brought forward. Those who have written on the subject have confined themselves to generalities. They have suggested the growth in the Cretaceous ocean of crops of sponges, and other silicious organisms. To account for their repetition at approximately regular intervals, no better suggestion has been made than that of Prof. Owen long ago. According to this view they are the remains of successive crops of sponges which grew again and again according to some periodic law. Assuming this—and it seems quite the most probable explanation yet offered—we have to ask, What was the length of the period, and what was the cause of the periodicity? First as to the length of the period between the successive crops of sponges. Prof. Soil as, in his estimation of the age of the earth, takes one foot in 100 years as the average rate of rock formation. But chalk is generally held to be a rock of much slower formation than many others. One geologist, indeed, suggests one foot in 1,000 years as the rate at which it was accumulated. Let us, then, take 6 inches of chalk in a century as a rough compromise. Since the remains of the successive crops, our lines of flints, are some three or four feet apart we have an interval of from six hundred to eight hundred years between each growth. A crop of sponges, then, flourishes for a certain time, dies, and is covered up with chalk. An interval of 600 years, or more, elapses, and a similar crop appears over the same area of the ocean. There may be even the same species of sponge in the new layer. But whether they are the same or new species, the difficulty is about the same. For 600 years there have been no sponges in this particular area of the chalk ocean. Where have the reproductive spores for the new crop come from? Not from the previous one. They could not have been floating about in the ocean all those centuries. The only possible suggestion is that they migrated away from the region, propagated themselves elsewhere for 600 years, and then floated back in the form of reproductive spores to their former home. These settling down produced the new crop of sponges, the succeeding line of flints. To be driven to such a suggestion is almost to confess a reductio ad absurdum. There seems to be absolutely no reason why the sponges should leave the area, and equally none for their coming back. And species of sponge coming back in that way would naturally do so gradually. They would not thus form a layer of flint keeping more or less to the same level in the chalk. At the periphery, where they entered the area, they would be lower in the chalk, and gradually rise. The flint layer would also be thicker in one direction, and thin out gradually. And what about the cause of the periodicity? Nature knows no periods of 600 years' duration. She celebrates no jubilee, as it were, nor does anything different from what she has been doing, at the end of such a time. To assume such periods is not geology, which is tied down to explain the past by the present. The only periods observed by nature effecting rock formation would appear to be annual. And such periods it must be supposed would leave their mark in the making of rocks. Thus the deposits of summer may differ from those of winter, or there may be seasonal pauses in sedimentation, which will leave their mark. It may be suggested that these marks of periodicity may often be read in the rocks themselves. If, then, the lines of flint in the chalk are periodical growths, must not the periods be years Must not each line of flint with its covering of three or four feet of chalk be the product of one year? A growth of sponges might take place during the summer months. As winter approached numerous free-swimming reproductive spores might be liberated, and the old sponges dying might be covered up. In the spring the reproductive bodies might settle down to produce a new crop. There is, at any rate, nothing impossible in such a course of events. But if so, we get a rate of rock formation of three hundred or four hundred feet in 100 years instead of one foot. Prof. Sollas's modest 26,000,000 years for earth history is reduced to 86,666 or 65,000. To most geologists, perhaps, this will appear as great a reductio ad absordum as the one indicated above. Yet it seems, on the assumption of periodic growths, the only possible solution. And after all, there is no certainty either in Prof. Sollas's 20,000,000, or in the 100,000,000 arrived at by other lines of investigation. These figures—and, indeed, much larger—are required by evolutionists, but they have not been proved by geology. And the case of the chalk flints does not stand alone in suggesting a quicker rata of rock formation than one foot in 100 years. Without here bringing forward details, it may be stated that every rock specially studied by the writer from this point of view seems to indicate a rate of formation much greater than one foot in 100 years. But the chief point it is wished to emphasize is, that every rock may be made to tell us something—quite roughly, of course, in many cases—of the time it took in the making, if carefully studied. And further, that in not a few cases the actual thickness formed in a year is made clear in the structure of the rock itself. A reconsideration of the one foot per century measure is required. Since writing the above my attention has been kindly called by Prof. Grenville A. J. Cole to a suggestion of his own on the subject of the origin of flint. “The rhythmic deposition of flint,” he says, “may be due to some action in a limestone mass in which the silica was at first uniformly diffused.” Prof. Cole also refers to the work of Liesegang on Geological Diffusion, on which his opinion was founded. And he further points out that Liesegang himself suggested an application of his experimental results to the explanation of chalk flints. “Liesegang,” says Prof. Cole, “compares the layers of flint with the zones of regular deposition that occur in cases of diffusion of one substance through another, and suggests that the silica in the chalk was formerly diffused fairly evenly, and that a progressive one-sided precipitation then took place.” The suggestion is an interesting one, and has a very important bearing on the question of the origin of chalk flints. I have not seen Liesegang's paper, nor do I know the details of Prof. Cole's suggestion, and so am not aware whether they consider this segregation of diffused filica to be sufficient in itself, without the periodic growth of sponges, to account for the facts. This is the important point as regards my original note. If it is put forward as the complete explanation of the occurrence of the flints in more or less regular layers I am afraid I cannot accept it. If it is to be considered as a supplementary agency I think it is absolutely necessary. So far as my chemical and physical knowledge carries me, it appears to me necessary to have some definite starting-points for the segregation of the diffused silica. A growth of silicious sponges would supply this. The silica of the sponge mass might start the process, and thus determine the “local habitation” of the flints. But it would require a considerable amount of diffused silica deposited on the sponges to produce a line of flints. Here, then, the suggestion of Liesegang and Prof. Cole would play its part. And there appears to be evidence that animal matter may start the segregation. For in the chalk there are frequently found the tests of sea-urchins and bivalve shells converted into solid masses of flint.—Science Progress. Immediately after the cutting-in-two process is finished. The “North Wind” with the hull parted amidships—showing the vertical cut Ready to begin the voyage through the Welland Canal. The stern section of the “North Wind,” afloat, boarded up and watertight
This article was originally published with the title "Chalk Flints and the Age of the Earth"