and its Bearing on the Coral Reef Problem. 195 



of elevation which raised them above the sea, the source of 

 their magnesian content. The evidence at Christmas Island is 

 very significant. Dr. Andrews* refers to the "thick beds of 

 nearly pure phosphate of lime, which cap several of the higher 

 hills. This substance is probably derived from ancient (? Plio- 

 cene) guano beds formed on the low islets which existed before 

 the first elevation of the island and is the insoluble residue of 

 beds of limestone altered by this guano." The significance of 

 this observation lies in the fact that immediately below the beds 

 of phosphate and in sharp contact with it, analyses by the writer 

 showed that the limestone had been completely dolomitized. 

 The inference is clear that this dolomitization took place in 

 very shallow water at the time when low islets rose above the 

 lagoon, and formed the resting place for birds to deposit guano. 

 This view of the shallow water origin of dolomite receives 

 support also from the occurrence of dolomitic limestones from 

 the raised island cliffs and terraces at Christmas Island and 

 Eva which are probably, but not quite certainly, raised fring- 

 ing reefs. 



In this connection it is significant as recorded by Waltherf 

 and Hume,:}: that in places, the upper raised fringing reefs of 

 the Red Sea have been dolomitized. The only movement 

 known to have affected this reef which was formed just below 

 sea level, is one of elevation. Its dolomitization must there- 

 fore have occurred under very shallow water conditions. The 

 writer^ believes he was the first to develop this hypothesis of a 

 very shallow water origin for "contemporaneous" or regional 

 dolomites. This view was amplified by him in a subsequent 

 paper, I and since then has been adopted bv several geologists. 

 The hypothesis can be further tested by considering the dis- 

 tribution of "contemporaneous" or "regional" dolomitization 

 among older limestones, and noting whether the parts which 

 are dolomitized are specially associated with shallow water 

 conditions as indicated by fossil content or special structures 

 such as false bedding, oolitic structure, conglomerates, etc. 



The association of dolomites in older limestones with independent 

 evidence of shallow water conditions. 



The chalk is a special limestone now generally believed to 

 have been formed at moderate depths. It is nearly everywhere 

 a fairly pure limestone with low content of MgC0 3 . The one 



*C. W. Andrews, Monograph of Christmas Island, p. 271. 

 t J. Walther, Abhandl. Math. Phvs. K. Sachs, Ges. Wiss., xiv, p. 494. 

 X W. F. Hume, The Rift Valleys "of E. Sinai, Int. Geol. Cong. Paris, 1900, 

 pp. 32-40. 

 SE. W. Skeats. Bull. Mus. Comp. Zool., Harvard, vol. xiii, 125, 1903. 

 ||E. W. Skeats, Q. J. G. S., lxi, pp. 131-137, 1905. 



