138 MISSOURI BOTANICAL GARDEN. 



in the valley of the Rhino. Todd * describes it as follows : 

 "Loess . , . is composed mainly of fine grains of silica 

 some of which attain the size of .2 nnn., though the majority 

 are less than .01 mm. These particles are coated, more 

 or less, with oxide of iron and calcic carbonate. The amount 

 of the former gives it the color which in the typical form is 

 usually a light buff. With it is mixed many small grains of 

 clay. The finer ingredients appear to constitute a cement 

 which holds it firmly together so that it usually stands firm 

 like indurated rock, unless softened by moisture." An in- 

 crease in the amount of oxide of iron increases the inten- 

 sity of the color of the loess. Its absence is indicated by a 



gray or even whitish color. 



3. Bouldery drift.— This underlies the loess, and covers 



below 



Mcramec 



strip two to five miles widc-f Interesting in this connection 

 is Wheeler's paper, bearing on the occurrence of blue gla- 

 cial clay or "till" at St. Louis. J From this it appears that 

 there exists some doubt as to the glacial origin of the drift 

 above referred to. However, 



more 



hed 



Subcarboniferous System. 



The ferruginous sandstone of the Chester group which in 

 other portions of St. Louis county underlies the coal measures 

 and in places attains a thickness of 250 feet, is, in our terri- 

 tory, represented by a thin layer of sandy shale, never more 

 than a few feet in thickness. It directly overlies the St. 

 Louis limestone. The latter is a very hard, fine-grained rock, 

 varying in color .from light yellow to light-gray and blue- 

 gray. It is evenly bedded, the strata varying in thickness 



dimension 



It is inter- 



stratifi(xl with layers of argillaceous shale. Some of the lime- 



* loc. cit. 



t Wright, G. F. U. S. Gcol. Surv. Bull. 58 : 72-73. 1890. 



t Wheeler, H. A. Note on the Glacial Drift in St. Louis. Trans. Acad. 



Sci. St. Louis. 7 : 121. 1S95. 



