ESSAYS 155 



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 sicccessive 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. Sollas, 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 in. 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 600 to 800 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 numerou s 



