58 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



most sandy and gritty layers is dolomitic. Hand-specimens representing the 

 principal phase? were collected; each of them has a cement soluble in hot 

 hydrochloric acid. The weathered surfaces of the arenaceous beds are always 

 roughened by the clastic grains of quartz and feldspar standing out above the 

 carbonate, the constituent more soluble in rain-water and soil-water. None of 

 the many specimens collected shows other than the feeblest effervescence with 

 cold, dilute acid. The specific gravity of thirteen specimens ranges from 

 2-688 in the most silicious phase, to 2-814 in the least silicious phase. The 

 average for all thirteen is 2-763. These facts, together with the character- 

 istic buff tint of the beds on weathered surfaces, of themselves indicated that 

 the formation is throughout highly magnesian. That conclusion has been 

 greatly strengthened by the chemical analysis of three specimens which, 

 respectively represent the staple rock-types in the lower, middle, and upper 

 members of the formation. The analyses will be described in connection with 

 the microscopic petrography of the three members. 



Lower Division. — Thin sections from the dominant rock of the lowest 

 member?, a very homogeneous, compact, thin-bedded limestone, show that the 

 carbonate occurs in the form of an exceedingly fine-textured aggregate of 

 closely packed, anhedral, colourless grains averaging from 0-01 mm. or less 

 to 0-02 mm. in diameter. The largest of the grains may run up to 0-03 mm. 

 in diameter. A very few minute, angular grains of quartz and unstriated 

 feldspar, and some dust-like, black particles (probably both magnetite and 

 carbon)- are embedded in the mass. The bedding is well marked in ledge or 

 hand-specimen but is yet more conspicuous under the microscope. The laminae 

 are bounded by sensibly plane surfaces, affording in section parallel lines 

 often only 0-2 mm. apart. This bedding lamination is brought out rather by 

 small differences of grain among the layers than by admixture of material 

 other than carbonate. 



The specimen chemically analyzed has the microscopic characters 

 just outlined. It was collected at the 5,050-foot contour on the spur running 

 southwe:-tward from the right-angled bend in Oil creek on the south side of the 

 creek and about one mile below Oil City. The analysis made by Professor 

 Dittrich (specimen No. 1322) showed weight percentages as follows: — 



Analysis of type specimen, lower Altyn formation. 



Mol. 



Si0 2 13-46 -224 



AL0 3 1-56 -015 



Fe 2 3 1-05 -008 



FeO -48 -007 



MgO 17-81 -445 



CaO 250,8 -448 



Na,0 -28 -005 



K 2 1-08 -012 



H 2 at 110°C -04 



H„0 above 110°C 1-23 -070 



C0 2 . 38-08 -865 



100-15 

 Sp.gr 2-805 



