NATURAL SCIENCES OF PHILADELPHIA. 



95 



has certain!}' a surface exposed, as the specimen which I have examined, has 

 been much weathered; the outer portion is partially reniform, which in places 

 shows a slight enamel, though not near as distinct as on the specimens from 

 Monk's and El Roque islands. It has evidently been formed in layers, which 

 when broken across show a slightly banded appearance not unlike that already 

 described as characteristic of portions of the rock from Monk's Island ; the 

 color varies from a buff to an ochrcous brown ; it is hard, brittle and much more 

 readily pulverized than the Monk's Islands guano rock. An opinion may seem 

 premature, but I am inclined to the belief that this has once been as the guano 

 rock from Monk's Island, which has undergone changes from causes not yet fully 

 studied, which seems ihore probable from the fact that small quantities of Phos- 

 phate of Alumina have been found in the Monk's Island rock. 



The whitest portion of the rock was selected for analysis, which gave the 

 following results : 



(VII.) 



1.5165 grams, ignited gave .3469 grams. 



" " 0.2000 grams, insoluble matter,* Silica, 



1.3048 " 



0.0474 

 0.6921 

 0.0720 

 .0.2120 

 0.1628 

 0.6438 

 0.0660 

 0.0057 



BaO, SO3 



2MgO, PO5 



CaO, CO2 " 



AI2O3 " 



Fe203 " 



2MgO,P05 " 



CaO, CO2 " 



2AI2 03,3P05?" 



gave Sulphuric Acid, 



" Phosphoric Acid, 



" Lime, 



" Alumina, 



" Sesquioxide of Iron, 



" Phosphoric Acid, 



" L(ime, 



22.87 perct. 



13.18 " 



1.07 " 



29.23 " 

 2.66 " 



16.24 " 

 12.41 " 

 31.60 " 



2.83 " 



Phosphate of Alumina, 0.43 



Water? 



Silica, 



Sulphuric Acid, 



Phosphoric Acid, 



Lime, 



Alumina, 



Sesquioxide of Iron, 



22.87 per cent. 



13.18 " " 



1.07 " " 



31.60 " " 



2.66 " " 



16.24 " " 



12.41 " " 



Oxygen ratio, 



21.54 

 6.84 

 0.65 



20.24 

 0.75 

 7.59 

 3.61 



The 0.65 of Oxygen of the Sulphuric Acid requires 0.22 of the Oxygen of the 

 Lime, the remaining lime is most likely combined with Carbonic Acid, as nearly 

 every specimen on treating with dilute acid gave a slight effervescence, (the guano 

 rock from the vertical cliff contained nearly one per cent.), or possibly a rem- 

 nant of the once phosphate of lime still exists. The alumina and phospho- 

 ric acid exist most likely in the form of Wavellite which has the formula 

 3AI2 O3 , 2PO54-I2HO. It is diflScult in a rock which is in progress of decompo- 

 sition to obtain other than approximate formulae. 



This island of Centinella is now inhabited by immense numbers of water- 

 fowl, many of which roost on the cliffs ; some of these roosts have been long oc- 

 cupied, and judging from the signs on the rocks beneath, have been much resorted 

 to. Mr. Richards of this city, who visited the islands a short time since broke 

 off a portion of the concretion which cover the cliffs beneath a bird roost of this 

 description ; the organic properties had mostly been removed by causes which 

 had converted the inorganic constituents into a guano rock, which in its com- 

 position, is not unlike that from Monk's Island and El Roque. Mr. Richards 

 mentioned that when first obtained it had quite a strong odor, which has now 

 almost disappeared ; it is very hard and was difficult to powder. 



The analysis of the concretion is as follows : 



1857.] 



