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 re- 

 specting 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 imme- 

 diately, without any passage-bed, upon the Gault. There is, how- 

 ever, 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 INeo- 

 comian 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 Plicatulce 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 Plicatulce may frequently be seen to be attached to 

 the broken surfaces. 



Moreover the matrix which constitutes the nodule-bed gives evi- 

 dence 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 de- 

 posit ; and these aro 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- 



