Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/97

1872.] FISHER — CRETACEOUS PHOSPHATIC NODULES. 55 phous lumps, varying from a very small size to pieces weighing about a quarter of a pound, though few are so large as that. The other variety, which, though numerous, is less so than the former, consists of long pieces, of such a form as would be produced by taking a cake of dough of the shape of an elongated ellipse, and folding it over a stick laid upon its longer axis. Sometimes the edges are closed, but often not so, and sometimes only partially. Frequently they appear to have been originally closed, but afterwards cracked open, owing to the contraction Caused by mineralization. The hollow axis of this variety is occupied by indurated marl with grains of chlorite. These elongated subcylindrical lumps attain a larger size than the other kind, often occurring of six ounces weight or more.

These two forms are alluded to by Mr. Seeley in his paper on the rock of the Cambridge Greensand*. He supposes them to have been formed from gelatinous phosphate of lime derived from seaweeds and decaying animals, and rolled on the shore into nodules, with carbonate of lime and the materials of the strand.

Mr. Bonney, in a paper read before the Geologists' Association†, considered the nodules to be mainly of concretionary origin; for they are too pure to be regarded as clay saturated with phosphate.

Upon examining a collection of the more unusual forms of the nodules, other very distinct varieties may be distinguished. It was when examining a set of these in the Woodwardian Museum more than a year ago that I first formed the opinion, from their arranging themselves so decidedly under what seemed to be species, that I had before me true fossil forms. There was one very perplexing specimen, of a cylindrical shape, and marked on the surface somewhat after the manner of the scales of a fir-cone. Mr. Walter Keeping told me that Mr. Jesson, of Trinity College, had a specimen of this, which showed it to be the interior of the fossil known to him as Scyphia tessellata. And upon examining Mr. Jesson's specimen I saw that this was undoubtedly the case. I was so far confirmed in my opinion.

I have had thin sections made of a few recognized organic forms, and also of the ordinary nodules, both amorphous and cylindrical.

The same general character of the matter of which they are composed prevails in all; and the manner of their petrifaction is alike. When seen by transmitted light they are of a more or less bright amber colour, or light brown; and when placed beneath the microscope appear minutely shaded with dark ramose lines, and speckled here and there with straight or very slightly curved spicular rods, sometimes with sides parallel, sometimes like the acute accent used in printing, and occasionally pointed at both ends. Although the surface of the nodules has a porous appearance, I have not been able to trace any canals connected with the seeming pores. They are traversed by shrinkage-cracks, which appear to reveal facts of importance towards interpreting other appearances.

When such a crack commences at the surface it is usually void for a small space, the soft matrix which occupied it having been removed in the preparation of the specimen. A little further in, it


 * Geol. Mag. vol. iii. p. 305.

† See abstract in Geol. Mag. vol. ix. p. 144.