REPOKT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 21 



all concretionary phosphates found united with the olivine rock of St. Paul Out of all 

 the specimens forwarded to me for inspection there was but one small fragment, a few 

 centimetres in size, which was coated with this substance. It had a lustrous glassy 

 appearance, like most natural and artificial scoriae ; but it obviously cannot be supposed 

 to have had an igneous origin. 1 Microscopical examination proves that it lacks the 

 characters invariably presented by natural glasses or fused minerals. 



I think it must be admitted that this concretionary substance was formed after the 

 manner of mineral incrustations. In aspect it is very similar to hyalite and other varieties 

 of colloidal silica, but it is neither so diaphanous, nor of such brilliant lustre as the former, 

 neither has it the hardness of opal ; then, again, its pyrognostic characters separate it 

 totally from all silicious incrustations. Darwin's description of it is very accurate. He 

 says that it is composed of very thin layers, spread out all over the surface of the rock, 

 and studded over with spheroidal forms having a concentric arrangement that at once recalls 

 the orbicular silicious concretions (orbicules siliceux of Brongniart). It is in perfect 

 contact with the surface of the rock, conforming with its every irregularity, and clinging 

 to it strongly. Barring its whitish tint, and bright lustrous appearance, I have been 

 unable to perceive any difference between this enamel and the concretionary phosphate 

 above described. The pyrognostic characters are the same as in the yellowish rosy mass 

 constituting the phosphatic veins and the breccias ; the structure is also the same in both 

 cases, and the chemical analyses prove the matter under consideration to be phosphate 

 of lime. 2 



I removed a very small portion of this incrustation, weighing about '0175 grammes, 

 and subjected it to a quantitative analysis. The quantity analysed was so minute that the 

 only certain results obtained gave phosphoric acid and lime, viz., P 2 5 , 33'61 per cent.; 

 and CaO, 50'51 per cent. Iron, magnesia, and sulphuric acid were also present. The 

 composition is, therefore, essentially a tribasic calcic phosphate, with sulphate of lime, 

 and perhaps also carbonate of lime, magnesia, and iron. 



The lithological description of the Rocks of St. Paul being now terminated, there 

 remains for inquiry the question whether as a whole the observations now obtained are 



1 Phipson (Journal Chem. Soc., xxv. p. 277 ; and Amer. Journal of Sc. and Arts, xxxvi. p. 423) gave the name 

 of sombrerite to a phosphatic rock which must possess great analogy with the material I am describing. He considers 

 it as composed of phosphate of lime, and phosphate of alumina. " It appears as an amorphous, gelatinous phosphate, 

 that has been subjected to the action of a high temperature." Julien (Amer. Journal of Sc. and Arts, p. 242) has 

 proved that Phipson's reasons for classing the sombrerite as a mineral are not well founded. He has shown that 

 it is a modem limestone formed at the bottom of the sea, and containing some guano, which occasionally yields 

 75 to 90 per cent of phosphate of lime, and from 3 to 4 per cent of carbonate of lime, 



* Piggot (Proc. Amer. PhiL Soc., Phila., vi 189 ; Am Journ. Science and Arts, 2nd series, 1856, No. 22) has 

 described a stony substance found at Los Monges, at the entrance of the Gulf of Maracaybo, and at other points along the 

 coast of the Carribbean Sea. At Los Monges it forms a very thin glossy layer over the deposits of common Mexican 

 guano. In some places the layer is an inch thick, in others it does not exceed a few millimetres. The chemical 

 analysis of this substance made by Higgin and Bickel shows that it consists of phosphate of lime and magnesia. I con- 

 sider the deposits at Los Monges as quite similar in their mode of formation to those of St PauL 



