PUBLICATIONS. 871 
and is rather easily eroded. There is no transition layer between the 
two, yet they interpenetrate somewhat. Processes of limestone sur- 
round portions of the shale, and lumps of limestone are found in the 
shale, moreover the limestone is roughly separated into beds by shale 
partings. They find that the steepness of the butte is a function of 
the rate of degradation of the surrounding surfaces, and that the height 
depends on the rate of degradation and the size of the cross section. 
~ As to cause, the facts do not seem to be sufficient as yet. They 
favor the idea that the limestone cores are due to colonies of Lucina, 
more especially which dying on these sites left their shells to form the 
limestone while fine silts were forming the shales around. Why the 
molluscs congregated at these places and what caused the apparent 
mortality we are left to conjecture. They are interesting topograph- 
ically and unique in some of their relations. 
RECENT PUBLICATIONS. 
—BECKER, GEO. F., “ Reconnaissance of the Gold Fields of the Southern 
Appalachians,” 85 pp. and 3 maps.—Sixteenth Ann. Report of the U. 
S. Geol. Survey, 1895. 
—BIRKENBINE, JOHN, “The Production of Iron Ores in various parts of 
the World,” 204 pp. and 6 maps.—Preliminary Brochure of the XVI. 
Ann.—Report of the U. S. Geol. Survey. 
. —BAIn, H. Foster, ‘Geology of Keokuk County.”—Geology of County, 
Makaska, Vol. IV. Geological Survey of lowa. 
“Essential Properties of Building Stones,’ 22 pp.—‘‘ Monthly Review 
of lIowa Weather and Crop Service,’ August and September 
1895. 
—BourRNS, FRANK S. AND WORCESTER, D. C. “Birds and Mammals 
Collected by the Menoge Scientific Expedition to the Phillippine 
Islands, 64 pp.—Minnesota Academy of Sciences, 1894. 
—CALAVERITE from Cripple Creek, Col., 4 pp—Am. Journal of Science, 
Vol. L., 1895. 
—CRAGIN, F. W., ‘““The Mentor Bed; A Central Kansas Terrane of the 
Comanche Series,” 4 pp.—American Geologist, Vol. XVI., 1895. 
. —Crossy, W. O., “Tables for the Determination of Common Minerals,” 
106 pp., Boston, 1895. 
—COoMISION GEOLOGICA MEXICANA Expediceén Cientifica al Popocata- 
petl, 38 pp. 6 plates and 1 map, Mexico, 1895. 
—Day, Wivti4M, C., “The Stone Industry in 1894,” 83 pp.—Preliminary 
Brochure of the XVI. Ann. Report of the U. S. Geological Survey. 
—DvumBLE E. J., “The Soils of Texas (a preliminary statement and classi- 
fication), 60 pp. and 1 map.—Texas Academy of Science. 
