350 



some shade of dark brown, and shows bands of very dark, alter- 

 nating with those of a hghter color. Like compact calcareous 

 concretions, the upper surface presents rounded elevations and 

 nodules, while below the mass is full of cavities and irregulari- 

 ties. Its fracture is generally splintery, and its average hardness, 

 greater than that of Fluor spar, is next to that of Feldspar. 



Sp. Gr. 2.440, (average.) 



The arenaceous guano may be considered as comminuted fish 

 bones, mixed with minute shells, still retaining organic matter ; 

 and one of the specimens shows the first step in aggregation, by 

 which solid masses form. These eventually by chemical operations, 

 become consolidated, so that the resulting body has all the char- 

 acters of a firm rock. 



It will be observed that the rounded grains of the arenaceous 

 guano, are generally of the size of mustard seed, and in forming 

 the sandstone aggregate, they show individually as the grains of 

 sand in that rock. In the guano-rock, this individuality is en- 

 tirely lost, and the eye detects nothing in the close-grained and 

 compact banded mass, which indicates its origin. Indeed, it 

 would be difficult to find two bodies mineralogically more diverse 

 than the two specimens before you. It is to this loss of the 

 granular form, and the production of a compact, remarkably 

 close-textured rock, that I wish to call your attention, as it has 

 an important connection with a subject which has long engaged 

 my leisure time, and which has been approached in some papers 

 I have before read here. 



This guano-rock has a composition not very different from 

 that of the arenaceous guano ; both, however, present a very 

 novel result, by analysis. In stating the composition, I pur- 

 posely omit several constituents which occur in minute quantity 

 only, and keep in view the bone phosphate of lime and organic 

 matter, as the prominent constituents of both. 



100 parts of the Guano Rock consist of — 



Moisture 0.80 



Dry Organic Acids, &c. 

 Sulphate of Lime .... 

 Bone rhosphate Lime and Magnesia 

 Sand . . . . . 



ILOO 



7.90 



110.20 



0.80 



130.70. 



