WCP4981

[1]¹ [p. 337]

Waldron Edge, Duppas Hill, Croydon,

23^rd November 1879.

James Croll, Esq.

My dear Mr. Croll,—I have been expecting with much interest to see your promised communication to Nature on the cause of the absence of change in the temperature of the equator at aphelion and perihelion.[2] [p. 338] I sincerely hope ill health may not be the cause of your silence on the subject. This, however, is not what I am now writing about. In the last few pages of my article in the Quarterly I touch very briefly on the great difficulty of the alternation of climate theory, the total absence of all indications of severe cold in the Arctic or Temperate regions, from the Miocene backward throughout all geological time. It is a difficulty that must not be shirked; for if the theory be the true one, I believe that some indications of the fauna and flora of these constantly recurring cold periods must exist, and can be found if sought for, especially during the later Secondary or earlier Tertiary formations. Your connection with the Geological Survey will probably enable you to direct me where to look for such indications with the best chance of finding them,—I mean in what works; or you may know some geologist who has worked at some extensive group of deposits in such detail as to be able to give the required information directly. It seemed to me that what is to be expected is, that in any extensive series of conformable strata of, say Eocene or Cretaceous age, there ought to occur at tolerably regular intervals a series of beds somewhat distinct in lithological character, and in which the usual fossils of the formation are either absent or partially replaced by dwarfed forms. Such beds might sometimes be very thin and insignificant, owing to the scanty deposition during an epoch when the bed was completely glaciated.

If any one such case can be found, either in this country or in a more northern latitude, it would give an immense support to the theory. You probably know if there are any of the geologists of the Survey who hold the theory firmly and have looked out for such evidences; or, if not, you can no doubt refer me to some one who can give me information on the subject, or to some work which gives the details of a series of beds and their fossils where such evidence is likely to be found.

Do not put yourself to any inconvenience to reply [3] [p. 339] to this letter until it is quite agreeable for you to do so.—Believe me, yours very faithfully,

Alfred R. Wallace.