branner: the stone reefs of brazil. 177 



silica; in another instance, the analysis showed 97 per cent of lime 

 carbonate and two per cent of silica.^ 



The analysis of the Brazilian reef rock, on the other hand, shows that 

 it is made up of about two parts of silica to one of lime carbonate, 

 including both cementing material and shell fragments. 



It does not seem improbable, judging solely from the amount of lime 

 carbonate in the rock, that the cementing materials may have come from 

 these same sands, having been dissolved from one part of the beds and 

 deposited in another. 



II. Lime carbonate from the ocean. — Hartt long ago suggested that 

 the lime carbonate by which the reef rocks are hardened was deposited 

 from sea-water.'* Such an explanation, however, can be looked upon as 

 satisfactory only when it is accompanied by some explanation of the 

 source of the lime, and the method and process of its deposition. It 

 may well be objected to tins theory that the beach sands in many parts 

 of the world are quite as calcareous as those of northeastern Brazil, that 

 the surf breaks upon these sands in the same way as it does upon the 

 Brazilian beaches, and yet they are not hardened by the deposition of 

 lime carbonate. Evidently a satisfactory explanation should deal with 

 this part of the problem. 



The theory of the process of hardening by lime carbonate from the 

 ocean water is briefly : that the carbon dioxide escapes where the surf 

 breaks upon the seashore, just as it escapes from streams at cataracts, or 

 wherever a disturbance throws the lime-charged waters into spray, or iu 

 any way lets the air at the water.^ Such surf should precipitate both 

 lime carbonate and iron, the latter being in solution as ferric car- 

 bonate, and being precipitated as feme oxide. The lime is thought 

 to be held in solution as a bicarbonate, and to be deposited as a lime 

 carbonate. 



What is the source of the carbonic acid necessary to form the lime 

 carbonate with which the Brazilian reef sands are hardened ? 



And why is this process of lithification not a universal one ] 



I conceive of two methods by which the carbon dioxide might come 

 from the ocean. 1. By submarine discharge of COo of volcanic origin. 

 2. COg derived from the sea-water itself. 



1 Amer. Journ. Sci., April, 1800; XXXIX., p. 240-256. 



2 Ch. Fred. Ilartt, Geology and pliysical gcograpliy of Brazil, p. 843. Boston, 

 1870. Amer. Nat., June, 1879, XIII., p. 347-358. 



^ George Murray, in his Introduction to the study of seaweeds, p. 8, points out 

 that air introduced into or passed through sea-water carries off carbon dioxide. 



VOL. XLIV. 12 



