L98 /*.'. II'. Skeats — The Formation of Dolomite 



same horizon- are much less dolomitized but it will be remem- 

 bered tliat these outcrops we further removed from the shore 

 line of the Carboniferous limestone sea" 



Later, on p. 17, he states, "Both groups of strata in which 

 contemporaneous dolomite occurs appear to have been formed 

 in shallow water. The Mumbles Head Beds throughout a 

 large tract of country contain a thin coal and marl with und-er- 

 clav which can only have been formed in the shallowest water, 

 while the "laminosa" dolomites may be inferred to have had a 

 shallow water origin from the fact that they are represented at 

 Pendine by a conglomerate which there is reason to believe 

 was actually a beach deposit." 



Dr. A. Strahan' x ' has described the lateral passage of a seam 

 of coal into dolomite at the Wirral Colliery in Cheshire. He 

 pictures both coal and dolomite as having been deposited be- 

 low sea level in quite shallow water. 



F. M. Van Tuylf in a paper entitled "A Contribtition to the 

 oolite problem" describes the sequence of the basal Ordovician 

 beds of northeastern Iowa as follows : — 



Prairie du Chien Dolomite 



Oolite 



Saint Croix Sandstone 



and states that the oolite is in places dolomitic, in places sili- 

 ceous. 



Steidtmann^: in his discussion of the dolomite problem states : 

 "Many dolomites have very obvious earmarks of shallow water 

 deposition, such as ripple marks, cross bedding, and interstrat- 

 ification with coarse sands." 



The above references to the occurrence of dolomites in the 

 chalk, the Trias, the Carboniferous and the Ordovician rocks 

 of different areas show that in each case there is independent 

 evidence of shallow water conditions. These consist of changes 

 in the organisms as in the chalk and Carboniferous Limestone, 

 the association with conglomerates as in the Trias and the Car- 

 boniferous limestone, the passage into a coal seam as in the 

 Carboniferous, and the association with oolitic structure as in 

 the Carboniferous limestone and the basal Ordovician series. 



It may be stated therefore that the modes of occurrence of 

 dolomite among the older sediments, so far as known to the 

 writer, yield powerful evidence in support of the hypothesis 

 of the shallow water origin of dolomite originally developed 

 by the writer from a study of its distribution among recent 

 and upraised coral limestones. 



* A. Strahan. Q. J. G. S., lvii, 297-306, 1901. 



\ F. M. VanTuyl, Journ. Geology, xxiv, 794, 1916. 



% E. Steidtmann, Jour. Geology, xix, 338, 1911. 



