74 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



squeezed or somewhat sheared, a cleavage was developed. Since the dip of the 

 bedding is generally low, this cleavage runs at high angles to the plane of 

 stratification. The cleavage planes have permitted easy passage to circulating 

 waters. Owing to their activity, there has been a wholesale segregation of the 

 more soluble lime-carbonate in the cleavages which have been thereby healed so as 

 to restore much of the rock's original strength. In regions of formerly strong 

 lateral pressure the rock is now converted into a laminated rock composed of 

 thin, alternating, more or less continuous layers of pure lime-carbonate. These 

 layers are highly inclined to the stratification planes and often run parallel to 

 cleavage planes in argillites above and below the limestone and, like those 

 planes, may be crumpled. (Plate 10 and Figure 9.) That the gray calcareous 

 partings are due to secondary chemical deposition is shown also by the fact 

 that where the original rock was argillaceous or sandy, these impurities remain 

 entirely within the magnesian parts of the rock. On a weathered surface the 

 latter may be quite gritty to the feel while the lime-carbonate partings are 

 smooth and marble-like. 



Bauerman described this rock as ' an impure limestone, in which the 

 carbonate of lime is intermingled with argillaceous patches in folds resembling 

 the markings in the molar tooth of an elephant.'* This appearance is most 

 striking on the weathered ledges, the stringers of the more soluble, gray calcite 

 locating numerous channels and pits which are separated by the brownish, pro- 

 jecting ribs of the more resistant magnesian and silicious parts. 



Using Bauerman's simile, the structure may be called the ' molar-tooth ' 

 structure, whereby will be understood, in general, the internal modification of 

 the original limestone by the secondary segregation of the calcium-carbonate. 

 The term will also be used for the very common case where the weathered 

 surfaces do not show the chance imitation of a worn molar-tooth; the last is 

 best shown in the cleaved phases. The name is thus conveniently generalized 

 as it may then be applied to the concretionary limestone even when cleavage 

 has not been developed. The structure is of importance as an aid in the 

 recognition of the Siyeh formation over great distances. 



Under the microscope the contrast between the calcitic and magnesian 

 parts of the molar-tooth rock is marked. The calcite, light gray in the hand- 

 specimen, is colourless in thin section. It forms a compact aggregate of poly- 

 gonal, sometimes interlocking grains varying in diameter from 0-005 mm. to 

 0-02 mm. and averaging about 0-01 mm. Very seldom, if ever, do these grains 

 show the rhombohedral or other crystal form. A few minute cubes of pyrite 

 are embedded in the mosaic, but, otherwise, the lenses and stringers are made 

 up of practically pure carbonate. 



The buff-weathering, main part of the rock is sharply distinguishable 

 under the microscope. It has a decided, pale yellowish-gray colour and a 

 mixed composition. Anhedra and rhombohedra of carbonate, which is doubt- 

 less high enough in magnesia to be called dolomite, form more than half of the 



♦Report of Progress, Geo]. Nat. Hist. Survey of Canada, for the years 1882-3-4, Pt. 

 B, p. 26. 



