Phosphatic 3Iatcrials used for Agncidtaml Purposes. 363 
appropriate name, for this material is not a guano deposit, but in 
reality the rock itself, of which the islet of Sombrero consists 
almost entirely. Although fragments of bones are not often 
found imbedded in the rock which is imported into England, it 
cannot be doubted that the latter is a true bone-breccia. I have 
in my collection a specimen of Sombrero Rock, in which several 
pieces of bone are distinctly visible. These pieces of bone are 
perfectly white, and free from organic matter. The rock varies 
considerably in colour, texture, and other external characters. 
Whilst some is porous and friable, other specimens are of con- 
siderable density. Most samples have a light yellowish-green 
colour, which is, however, sometimes varied by a bright green 
or bright yellow, a violet, bluish, or pinkish hue. On the 
whole, this rock is reduced to powder with tolerable ease. 
I have carefully and minutely examined some average sam- 
ples taken from a bulk of several tons, which gave the following 
results : — 
Composition of Sombrero Rock or Crust Guano. 
Moisture 
Water of combination 
and a little organic 
matter 
*Phosphoric acid 
Lime 
Magnesia 
Alkalies and fluorine 
Oxide of iron . . 
Alumina 
Carbonic acid . . 
Sulphuric acid . . 
Chlorine 
Insoluble 
matter 
siliceous 
No. 1. 
9*06 
4-38 
34-41 
36-17 
-36 
1- 86 
2- 82 
6-89 
1-55 
-66 
Notdeter- 
mined. 
1-84 
' Equal to tribasic phos 
phate of lime (bone- 
earth) . . 
100-00 
74-55 
No. 2. 
7-51 
•19 
09 
19 
44 
87 
22 
26 
36 
■44 
•39 
1-04 
100-00 
76-02 
No. 3. ! No. 4. 
10-09 i 4-22 
4-90 
34-11 
38-42 
•41 
1- 61 
2- 85 
4-23 
1-68 
•36 
1-03 
100^00 
73-90 
6'57 
34-76 
39-07 
•52 
1- 85 
2- 98 
6-23 
1-75 
•36 
•28 
1-41 
100-00 
75-31 
No. 5. 
2-94" 
5-941 
2^05 
100-00 
76-90 
No. 6. 
100-00 
69-42 
No. 7. 
15 
10 
13-08 
32 
51 
34-34 
35 
95 
37-52 
2 
74] 
11-73 
11 
42 
1 
14 
1-45 
1 
14 
1-88 
100-00 
74-40 
Sombrero Rock has been used in America for agricultural 
purposes, it is said, with considerable success, when simply 
reduced to powder. Such a practice cannot, however, be recom- 
mended ; for this, like most mineral phosphates, requires to be 
treated with sulphuric acid in order to become really efficacious 
as a manure. It will be noticed that there is not much carbonate 
of lime, but a good deal of alumina, and in some samples also a 
good deal of oxide of iron. The proportion of phosphate of lime 
VOL. XXI. 2 C 
