Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/160

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work on the Geological Survey in the Crag District has led me to think that previous observers have made a slight error in the classification of a certain ferruginous sand that is often to be seen above the shelly Red Crag, the line of junction being mostly very irregular.

This sand has been described by Prof. Prestwich as an "upper division" of the Red Crag, or, to quote his own words, "owing to the want of all fossils in the neighbourhood of Ipswich, as the 'unproductive sands' of the Red Crag" ; and he goes on to speak of the "erosion of the lower division" underneath this, afterwards classing the upper with the Chillesford sands.

Mr. S. Y. Wood, Jun., has referred the ferruginous sand in question to various horizons in the Glacial Drift, I believe with a constant tendency to lower its horizon; but his former views need not be dwelt upon, as that tendency has continued until he has accepted my classification, and now regards this sand simply as Red Crag, not separated stratigraphically or palæontologically from the shelly mass below.

The so-called "eroded" surface of the shelly Crag, noticed by various observers, is, indeed, apparently so only; but I must say that in many sections there is little or nothing to throw doubt on the reality of the appearance, which is somewhat analogous to the "mimicry" sometimes seen in insect life, though in our case one cannot see any object to be served by the delusion, unless it be the bewilderment of geologists. An examination of a large number of sections, however, and an attention to mere local details that could hardly be expected from any one but an observer who is obliged to note them as a matter of business, has conclusively shown that we have in this case not an eroded surface, worn out in a lower before the deposition of a higher bed, but an occurrence akin to the "pipes" of sand &c. so often seen piercing the top of the Chalk, and which, too, have also been taken as evidence of erosion, though their origin is now well understood: we have, in fact, an irregular underground surface, caused by the dissolving action of carbonated water in permeable beds, a surface formed after the deposition of the upper beds by the dissolving away of the shells that they once contained.

That the above is the true explanation of the irregular removal of the shells was first suggested to me by the fact that the apparently unfossiliferous sand above is, for the most part, exactly like the sand of the shelly Crag below, differing only in the absence of shells. Confirmatory evidence was given by the not uncommon occurrence in the upper sand of layers or masses of ironstone or