﻿Vol. 6 1.] DOLOMITES OF SOUTHERN TYROL. 133 



to the operation of the forces of segregation, forces which are 

 concerned in the formation of concretionary structures like the 

 septaria in the London Clay and the ' doggers ' in the Oolites. 



Prof. Garwood, 1 in 1891, published the results of his in- 

 vestigations ' On the Origin & Mode of Formation of the Concre- 

 tions in the Magnesian Limestones of Durham.' He showed that 

 certain bands in the rock have practically the composition of 

 dolomite, while in other layers, by subsequent solution, segregation 

 of the calcium-carbonate has been set up, and concretionary struc- 

 tures are formed which consist mainly of calcium-carbonate. Prof. 

 Garwood's results are interesting in this connection, because the 

 effect of the segregative forces in the formation of the concretions 

 is not the enrichment by magnesium-carbonate of a slightly- 

 magnesian limestone, but the dedolomitization of a rock which 

 formerly had almost the composition of a dolomite. Under other 

 conditions than those obtaining in the Magnesian Limestone of 

 Durham, it is quite possible that the calcium-carbonate may be 

 removed altogether by solution, and that segregation of the re- 

 maining mass will give rise locally to the formation of dolomite. 

 In some localities where dolomitic limestones are found, their 

 occurrence is often of a patchy character, the dolomite disap- 

 pearing horizontally and vertically in a short distance. It may 

 very well be that some of these occurrences of dolomite could be 

 best explained by a process of segregation, especially as in general 

 the effects of the segregative forces (as exemplified by septaria 

 and ' doggers ') are of a quite local character. 



In the Dolomites of the Tyrol, however, the dolomitization is 

 not local, but on a very extensive scale. The Schlern Dolomite 

 extends over many square miles, and in places exceeds 3000 feet 

 in thickness, so that no local cause can explain its production over 

 such large areas. The rock was without doubt originally a lime- 

 stone, composed entirely of organisms, and was subsequently con- 

 verted into dolomite. There can be no question that the magnesium 

 was obtained from the sea-water. Chief interest is centred in the 

 conditions under which this partial replacement of calcium-carbonate 

 by magnesium-carbonate took place. 



The Tyrol rocks are so completely dolomitized that they do not, 

 of themselves, afford much indication of the conditions under which 

 they were formed. Examination of specimens from upraised coral- 

 islands, such as Christmas Island, the Fijis, etc., led me to the 

 conclusion that the formation of dolomite can proceed in 

 quite shallow waters. 2 In Christmas Island a band of dolomite 

 occurs immediately below beds of calcium-phosphate, which cap the 

 highest points of the island. These were low islets in the ancient 

 lagoon, on which bird-droppings fell. The occurrence of dolomite 

 immediately beneath these phosphate-beds points to its formation 

 in superficial waters. Many of the Fiji islands are also dolomitized 



1 Geol. Mag. 1891, pp. 433-40. 



2 Bull. Mus. Comp. Zool. vol, xlii (1903) p. 125. 



