Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/95

1872.] FISHER — CRETACEOUS PHOSPHATIC NODULES. 53 observation by Prof. Phillips, that our ignorance of the origin of the phosphatic nodules occurring at many horizons was a reproach to geology. A very short time afterwards the subject was proposed for the Sedgwick prize at Cambridge for 1871; but no essay was sent in. I live in the midst of the so-called Coprolite pits of Cambridgeshire, and have had my attention directed to them continuously for some time, and in May last I communicated to the Society a paper respecting them, with diagrams kindly drawn for me by Mr. Martin, of Christ's College. In consequence, however, of information which has since come to my knowledge, I was led to obtain permission to withdraw this paper, which I now offer afresh, with such alterations and additions as seem to be required.

The Cambridgeshire phosphatic nodules, as is well known, are extracted by washing from a stratum (seldom much exceeding a foot in thickness) lying at the base of the lower chalk, and resting immediately, without any passage-bed, upon the Gault. There is, however, a gradual passage upwards from the nodule-bed into the lower chalk or clunch. The average yield is about 300 tons per acre; and the nodules are worth about 50 shillings a ton. The diggers usually pay about £140 an acre for the privilege of digging, and return the land at the end of two years properly levelled and re-soiled. They follow the nodules to a depth of about 20 feet; but it scarcely pays to extract them to that depth. At some future time mining may be resorted to; but there will be some difficulties, on account of the abundance of water which is held up by the Gault, and on account of the loose texture of the clunch above.

A feature common to our nodule-bed and to those of Suffolk in the Crag, and of Potton and Wicken in the Neocomian, is that they are derivative beds. In the case of the Crag and of the Neocomian beds, fossils of several periods are mingled. This is not the case in our cretaceous nodule-bed, where all the organic fossils appear to belong to the Lower Cretaceous period.

Among the derived fossils of this deposit are the greater part of the nodules themselves; although I believe a few of them to be strictly in situ. All the derived specimens have Plicatuloe attached to them, showing that, whatever they were originally, they were hard bodies when they lay at the bottom of the Chalk ocean. Many of them are broken; and the Plicatuloe may frequently be seen to be attached to the broken surfaces.

Moreover the matrix which constitutes the nodule-bed gives evidence of being drifted. It contains small lumps of Chalk-marl which have fewer green grains in them than the matrix in which they are imbedded. The green grains are also evidently drifted, and dispersed in patches and layers through the deposit, having been apparently accumulated by washing from a cretaceous rock in which they were more sparsely present.

There are only certain calcareous organisms preserved in this deposit; and these are of the same kinds that we usually observe to have escaped destruction in beds which have been unfavourable to the preservation of shells. Other mollusks are found only in the con-