L88 E. W. Skeats — The Fur ma lion of Dolomite 



Van Tuvl.-' "Dolomite lias been frequently prepared artificially 

 under conditions of high temperature and high pressure or 

 both but it lias been produced in the laboratory at ordinary 

 temperature and pressure only in rare instances and then in 

 minute amounts and under conditions winch doubtfully operate 

 in nature, at least on a large scale. It must be conceded then 

 that these experiments furnish little evidence as to the actual 

 conditions obtaining when extensive beds of dolomite are 



formed naturally." 



i 



The chemical precipitate hypothesis of the origin of dolomite. 



As pointed out by Van Tuylf many geologists have advo- 

 cated the view that dolomite is a direct chemical precipitate, 

 including Bone, Bertram-Geslin, Coqnand, Zirkel, Fournet, 

 Loretz, Forchammer, Hunt, Vogt, Daly, Linck, and Suess. 



Some of these writers have been led to this conclusion by the 

 existence of fine-grained structureless dolomites apparently 

 devoid of organisms. This conclusion may be partially correct 

 in the case of certain dolomites formed in restricted seas under 

 conditions of concentration and this view may apply to the 

 formation of parts of the magnesium limestone of the North 

 of England, to parts of the Raibl dolomites in the Tyrol J and 

 some dolomite-bearing rocks in the Triassic rocks of Britain. 



Dr. Cullis,§ for instance, has explained the presence of 

 minute rhombohedra of dolomite in some of the Keuper marls 

 of Britain as being due to direct chemical precipitation. 



Modern work on the limestones and dolomites of coral 

 islands has shown however|| "' that fine grained structureless 

 dolomites occur among reef limestones, and have arisen by 

 complete recrystallization and metasomatic alteration of lime- 

 stones originally composed of calcareous organisms." 



Zirkel's"([ observations that crystals of dolomite occurring in 

 veins and druses indicate its possible chemical deposition on a 

 larger scale in nature is not borne out by the detailed examin- 

 ation of coral island limestones made by Dr. Ouliis** and the 

 writer, ff who found at Funafuti and among upraised coral lime- 

 stones respectively that while primary deposition of dolomite 

 occurs, it appears to be limited to the formation of small quan- 

 tities of dolomite occurring as rhombohedra lining calcite 

 crystals in cavities in the limestone or as occasional layers alter- 



* Op. cit. t Op. cit. 



X Skeats, op. cit. 



SOallis, Rep. Brit. Asso., 1907, pp. 506-507. 



|| Skeats, Bull. Mus. Comp. Zool., Harvard 1903; Cullis, Funafuti Reports, 

 London 1904. 



If Zirkel's Lehrbuch der Petrographie. 2nd ed., vol. ii, p. 503. 

 **Op. cit. tfOpcit- 



