REPORT OF THE CHIEF ASTRONOMER 73 



SESSIONAL PAPER No. 25a 



arly, sometimes systematically. The buff-tinted (weathered) general surface 

 of the ledge is thus variegated with many small masses of pure light gray to 

 bluish gray limestone. These masses are in the form of roundish nodules and 

 pencils, flat lenses, or irregular stringers of no definite shape. They are essen- 

 tially composed of pure calcite; they effervesce violently with cold dilute acid. 

 As in so many dolomites the calcareous segregation is often quite unsystematic, 





M 



M 



\ 





M 





-\ ,■■'. 



\\N ^ A ^ ^ 



X 



Iigure 9. — Section showing common phase of the molar-tooth structure in theSiyeh 

 formation. The calcicic segregations are lenticular and stand perpendicular to 

 the plane of stratification, as shown by the metargillitic intercedes (M). The 

 middle layer of limestone is two feet thick. Locality, north fork of the Yahk 

 River. 



but, on the average, it is definitely related to the two master planes of structure 

 in the limestone. Where the rock is uncleaved, the bedding-plane has been 

 selected .as the favoured locus of growth of the segregation. The stratification 

 may thus be marked by many small, independent lenses of lime carbonate com- 

 pletely surrounded by the magnesian matrix. There is transition between such 

 isolated lenses and entire, uninterrupted beds of gray limestone conformably 

 intercalated in the buff magnesian rock. Such beds may in many cases be due 

 to original sedimentation. 



The conclusion that the lime-carbonate lenses, pencils, and irregular bodies, 

 and even some of the continuous bed-like masses, are due to secondary segrega- 

 tion within the dolomite, is clearly upheld by the relation of ,another kind of 

 lime-carbonate partings. These were long ago observed by Bauerman and later 

 by Willis. In localities where the dolomite has been specially nipped and 



