57 
element in rocks, * it is I believe generally only present in 
very small quantities. It occurs native for example as a 
white, amorphous mineral, known under the name of 
Phosphorite. It also enters largely into the composition of 
the bones of animals. Probably, therefore, a large proportion 
of these concretions, both in the Crag and Green Sand, were 
obtained chiefly from the decomposition of animal remains, 
and partly perhaps from the destruction of rocks containing 
Phosphorite, and in some instances from vegetable matter. 
Indeed, Mr. Seely is of opinion that the Cambridge nodules 
were derived entirely from Zestera and other marine plants, 
although I think this is a very questionable source, to the full 
extent here suggested. Mr. Walker states that the concre- 
tions in Bedfordshire contain a much larger per centage of 
Alumina than those in the Green Sand in Cambridgeshire. 
This he thinks indicates that they had been formed of clay, 
soaked in decomposing animal and vegetable matter, since 
the alumina could not be derived from either animal or 
vegetable sources. 
The following is an analysis of some of the best average 
samples of phosphatic nodules from Bedfordshire, made by 
Dr. Voelcker. f 
Water of Combination ae Bette icon SOT 
Phosphoric Acid { to Re eo. sas Locka. 
Tames ="... os ae 32.73. ... 26.69. 
Magnesia, Alumina, and Flourine, 
(by difference.) ... ww» 6.64. 1... 4.51. 
Carbonic Acid § ... aoe DeOGs Sent eae 
* In a Paper, by Dr. Voelcker, read at the British Association, in 1865, he des- 
cribes certain limestones and black shales in the Llandirlo series, (Lower Silurian,) 
in North Wales, which are rich in phosphate of lime; in one case the proportion 
amounted to 643 percent. The mine he states contains many millions of tons of 
valuable phosphatic minerals. 
t+ See my Paper on the Green Sand, at Sandy. Geological Magazine, egies re) 
{ Equal to Tribassic Phosphate of Lime, (Bone Earth) 48.51. be 
§ Equal to Carbonate of Lime oat aaa wwe 6,95. ore “e os 
e 
