E. Blackwelder—Reef of Calcareous Algce (f). 649 



somewhat mottled in a very indefinite fashion with paler gray. 

 The texture is line and relatively uniform, and there is no dis- 

 tinct trace of original structure. The mortar-like matrix, seen 

 on the outside, is of a much lighter gray tint, and likewise con- 

 sists of dense, fine-grained dolomite without mottling. 



The occurrence of these hemispherical bodies at a definite 

 horizon, bottomed on limestone, suggests a reef of colonial 



Fig. 3. 



Fig. 3. Drawing, made in the field, to show the internal structure of one 

 of the domes that had been split open by frost action. 



organisms, such as corals, hydroids, bryozoans or algae. The 

 concentric internal structure of each mass supports the hypothe- 

 sis that it originated by a process of growth upward and out- 

 ward from a center. From the fact that the only parts of the 

 mass now consisting of lime carbonate are disconnected bodies 

 and layers near the center, it is inferred that although origi- 

 nally a lime-carbonate structure, it has been dolomitized from 

 the outside inward, by a magnesia-bearing solution. The prob- 

 ability that this pseudomorphic alteration has affected many 

 limestones,* either reef-rock or of other origin, renders it 

 unnecessary to argue the question at length in this case. To 

 this process may be ascribed the lack of recognizable coarser 

 organic structure throughout the dolomitic part of the mass. 



* See the following : 



Skeats, E. W. : Chemical and Mineralogical Evidence as to the Origin of 

 the Dolomites of southern Tyrol, Quart. Jour. Geol. Soc, vol. lxi, pp. 97- 

 139, 1905. 



Steidtmatm, E. : The Evolution of Limestone and Dolomite, Jour, of Geol., 

 vol. xix, pp. 323-345, 392-428, 1911. 



Blackwelder : Origin of the Bighorn Dolomite, Bull. G. S. A., vol. xxiv, 

 pp. 607-624, 1913. 



Wallace, B. C. : Pseudobrecciation in Ordovician Limestones in Manitoba, 

 Jour, of Geol., vol. xxi, No. 5, pp. 402-421, 1913, and many others. 



