PETROGRAPHICAL CHARACTER OF THE KOXA DOLOMITE. 247 



largelv rlionihohedra. These very finely granular rocks vary into those 

 which are more coarsely crystalline, and the latter grade into phases in 

 which coarse crystals of dolomite compose most of the rock. It is probable 

 that in these coarser rocks there has been a recrystallization. This is indi- 

 cated in one case by a remarkably beautifnl zonal structure, shown by all 

 of the large rhombohedra of dolomite. 



The purer phases of dolomite pass into those in which the dolomite 

 is heavilv stained with iron oxid'e. In some cases on the weathered surface 

 is an outer layer of heavily ferruginous material, resulting from the decom- 

 position of the dolomite, and showing that the carbonate is ferriferous. 



These noufragmental rocks by gradation pass on the one hand into 

 the argillaceous dolomites or mica-slates and upon the other into the 

 quartzose dolomites. In the argillaceous dolomites the finely crystalline 

 quartz, feldspar, and other clayey materials are intimately intermingled 

 with the granules of dolomite. By a further decrease of the dolomite the 

 rocks pass into the dolomitic slates. Those which show the fragmental 

 material in a dolomitic liackground are placed with the dolomites. Those 

 Avhich show a fragmental background in which the dolomite occurs are 

 placed with the slates. Where the detritus is coarse the rocks are qiiartzose 

 dolomites. In these we have a large amount of fragmental (piartz, in well- 

 rounded, enlarged grains. Where the quai-tz grains are buried in a back- 

 ground of dolomite the}' are called siliceous dolomites. By a decrease of 

 the dolomite we have a sparse matrix of carbonate in Avhich numerous 

 quartz grains are set, and then the rock becomes a dolomitic quartzite. 

 In some cases the alternations of coarse and fine material are in minute 

 layers, a fraction of an inch across, having alternately coarse and fine 

 grains of quartz and greatly varying amounts of dolomite. In other cases 

 thick beds are wholly of the dolomitic quartzite. 



The rocks of the formation, whatever their lithological character, have 

 been shattered by dynamic action, and have frequently become reibungs- 

 breccias (PI. VII, fig 2, and PI. VIII) These breccias, which where much 

 mashed resemble true elastics, diff"er from conglomerates in the usual angu- 

 larity of the fragments and in containing no material from an extraneous 

 source. 



