16 IOWA ACADEMY OF SCIE^XES. 



4. Taking a geceral view of the formations, there seems to be a slight anti- 

 clinal axis, trending in a northeasterly direction. North of this a broad depression 

 in which, as said before, the firmer cretaceous rocks sink below the Big Sioux. In 

 the vicinity of the red quartzite the cretaceous beds rise again to prominence. In 

 this basin considerable thickness of lignite is reported in the vicinity of Center- 

 ville, S. D. Water has prevented an opening of the beds which are said to be 

 4 or 5 feet thick ami within ICO feet of the surface. 



5. It is an interesting fact that the cretaceous clays and chalkstone are usually 

 attended by MenlzoUa ort.aia, Shtphodia atgeniea and Schrank'm uncitiata. In 

 fact the last has often disclosed to me the cretaceous character of a slope, which 

 otherwise might have passed unnoticed. 



EXHIBITION OF VOLCANIC DUST FROM OMAHA, NEBRASKA. 



This material was from a stratum of whitish aspect, about 18 inches in thick- 

 ness, found in the bluffs facing the Missouri river about 1}4 miles north of Omaha. 

 It has the same general characteristics as the volcanic dust which has been found 

 in quantity along the Republican, m southern Nebraska, also in Knox, Cumming 

 and Seward counties in the same State. This statement is made on the authority 

 of J. S. Diller of the United States Geological survey, who has examined samples 

 from all these localities microscopically. This ditfers in being stained with oxide 

 of iron, and the sharp angular grains are coated with carbonate of lime. Like the 

 rest it contains with the finely pulverized glass, a tew rounded grains of quartz, 

 and angular grains of feldspar less than .02 of a millimeter in diameter. The 

 dust is such as is carried through the air from volcanoes. The sand grains and 

 occasional diatoms indicate its deposition in still water. 



The lollowing is a secticn of the bluff containing the volcanic dust stratum: 



Twenty-five to thirty feet — Loess, exposed as much more on slope above. 



Seven feet— Stratified yellow clayey loam, with many calcareous concretions. 



One and one-half feet — Volvanic dust, stained with iron oxide. 



Five feet — Yellow clayey loam, slightly stratified. 



One-half toot— Fine gray sand. 



Twenty leet — Coarse sacd and pebbles obliquely stratified. 



Fifteen feet— Unknown, probably in part blue till. Level of the Missouri river. 



This locality is the most eastern exposure of the volcanic dust stratum which is 

 found scattered over most of Nebraska. Dilligent search has as yet failed to dis- 

 cover it on the Iowa side of the Missouri. 



