172 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Analysis of Rock from the Stoxe Reef of Rio Formoso, 

 R. N. Brackett, Analyst. 



Specimen dried at 110° to 115° Centigrade. 



Per Cent Per Ceat 



Matter soluble in coucentrated hydrochloric acid 35.94 



Matter insoluble in concentrated hydrochloric acid 64.06 



Silica (SiOg) (nearly all quartz sand) . . . 63.52 



Alumina (AlgOg) 0.39 



Iron (FegOg) 0.36 



Lime carbonate (CaCOg) 29.65 



Carbonate of magnesia (MgCOg) 4.97 



Phosphoric anhydride (P2O5) trace 



Magnesia (MgO) 0.18 



Potash (K2O) 0.21 



Soda (NaaO) 0.28 



Loss on ignition 0.39 



Total 99.95 100.00 



The analysis shows a rather high percentage of lime, but this lime 

 does not all come from the hardening material between sand grains. 

 The rock contains many pieces of shells, corals, and other calcareous 

 fragments, in addition to the lime deposited between the sand grains ; 

 it is not possible to entirely remove all of these shell fragments from 

 the material used in the analysis. 



An analysis was made by Prof. L. R. Lenox of the binding material of 

 the Mamanguape reef rock. In order tliat the results of this analysis 

 may be compared with that of the Rio Formoso reef rock, the two are 

 put here side by side. 



Analyses of the Binding Material of Stone Reefs. 



Rio Formoso rock. Mamanguape rock. 



Lime (CaO) 16.60 17.59 



Magnesia (MgO) 2.55 2.58 



Equivalent to carbonate of lime (CaCOg) 29.65 31.41 



Carbonate of magnesium (MgCOg) . . 4.97 5.39 



There is, therefore, a striking similarity in the binding materials of 

 the two rocks, though the specimens came from reefs more than two 

 hundred kilometres apart. 



The hardening of the rock has been produced by the deposition of 

 lime carbonate and magnesium carbonate in the sands. When placed 

 in acid, the carbonate is soon dissolved, and the sand grains fall apart ; 

 second, a microscopic examination of thin sections of the rock shows 



