general account of the deposits. 9 



The Phosphatic Nodules. 



The phosphate of lime nodules are mostly of small size, ranging 

 from mere grains up to masses equal to the double fist. They 

 consist of a dull compact material, the better kinds having a cubical 

 fracture and bearing much resemblance to chocolate cake both in 

 colour and texture. They are about as hard as ordinary limestone, 

 or septarian nodules. 



The colour varies from dark chocolate brown to a pale creamy 

 yellow, these variations depending in part upon the nature of the 

 original rock from which the nodule was derived, but not entirely 

 so, since it is a general rule that the darker 'coprolites' are found 

 towards the bottom of the pit. The darker nodules are said 

 to be richer in phosphate of lime; other varieties of the phosphatic 

 nodules are produced by the varying admixture of sandy matter in 

 them, so that we get all the different degrees of purity from the 

 perfectly homogeneous and compact phosphate, through specimens 

 with more and more abundant sandy particles till we get a very 

 impure phosphatic grit. Mr Walker has pointed out to me that 

 many of the Portlandian species {Cardium dissimile and the Tri- 

 gonice) are commonly found in a dark gritty variety of phosphate, 

 but I am not able satisfactorily thus to separate out the phospha- 

 tised fossils into groups of different ages according to differences in 

 the matrix. 



The phosphatic nodules are either fragmentary portions of 

 larger masses or the casts of fossils. In the latter case the ex- 

 posed outlines are, as a rule, thoroughly worn into rounded surfaces 

 of abrasion, so that the original shape is scarcely, or not at all, 

 recognisable; they are also much tunnelled by numerous boring 

 organisms. These stone-borers were for the most part sponges, 

 worms, and bivalve shells, especially the latter, whose crypts often 

 cover the whole surface of the nodule, leaving the outer zone of 

 the 'coprolite' thoroughly honeycombed. But besides these there 

 are other small markings of very general occurrence upon and 

 throughout the phosphatic nodules whose nature is far more 

 problematical ; namely, certain curious branching, interlacing, 

 undulating or simply straight-crossing structures forming little 

 gutters over the surface of the nodule, and canals penetrating into 

 its substance. Some of these are mere shrinkage cracks and 



k 



