JANUABY 15, 1915] 



SCIENCE 



107 



above sea. As mapped, it lies at all altitudes 

 between 380 and 500, and there are somewhat 

 flattish areas at all altitudes, but especially at 

 440^50 the altitude of the top of the valley 

 filling in this region. Since divides generally 

 have a rounded profile, and those of this area 

 stand about 500 feet above sea, probably more 

 of the surface here is near that altitude than 

 any other, but the fact is scarcely evident from 

 the topographic maps. 



Also the loess does not seem to the writer 

 to show a marked change in character at the 

 500-foot contour. In color, texture and other 

 physical and chemical characters, including 

 those brought out by mechanical analyses, acid 

 tests and the microscope, the material seems to 

 be a single deposit. In a very rough way the 

 altitude of the surface increases with distance 

 from the river and for this reason the highest 

 " marl-loess " may be, on the whole, a little 

 more like the clayey phase ordinarily found 

 at some distance from larger streams than the 

 average bluff loess, but the difference is slight 

 and there seems to be no noticeable change at 

 the 500 contour. 



The statement that the range of fossils " is 

 coextensive with that of the marl-loess, none 

 being found above the 500-foot level " is not in 

 accord with the writer's observations. One 

 rather large collection was made at an alti- 

 tude of 555 feet, two miles north of the center 

 of Patoka and another at 525, one fourth of a 

 mile northwest of that point. Another collec- 

 tion was made two miles north and one and 

 one half miles west of Owensville at an alti- 

 tude of 535 feet. At the first named locality 

 the following species, all of which are land 

 shells, were collected: Pyramidula alternata 

 Say, Succinea sp. (young), Helicodiscus multi- 

 lineatus Say, Euconulus trochiformis Mon- 

 tague, Polygyra monodon Hock, P. hirsuta 

 Say, Pupa muscorum L., Helicina occulta Say. 



The maximum thickness of the " marl- 

 loess " is only about 40 feet, and yet it is con- 

 tinuous except where it has been subjected to 

 severe erosion, as on the steeper valley sides. 

 If the " marl-loess " were water deposited — 

 a valley filling now dissected — one might rea- 

 sonably expect to find remnants of triangular 



cross section and limited extent located where 

 the meandering streams had chanced to leave 

 them, instead of an almost continuous layer 

 covering valley sides. The " marl-loess " as 

 mapped has a vertical range of 120 feet or 

 more, but the maximum thickness is scarcely 

 a third as much. Like true loess, it appears to 

 mantle hiU and valley alike, in places oblit- 

 erating minor irregularities, but nowhere 

 greatly modifying the major features. 



The writer found no pebbles in place in the 

 material which he would class as loess, and 

 none in any of the material described as marl- 

 loess at a greater altitude than 440 or at most 

 450 feet. Pebbles are, to be sure, pretty good 

 evidence that a deposit is not eolian, provid- 

 ing they are certainly in place. But there are 

 so many ways in which pebbles are scattered 

 that unless they were certainly in place or 

 very nearly in place, it would seem unsafe to 

 regard them as evidence of water deposition. 



The relation of the true loess to the gen- 

 eral form of the surface of southern Illinois 

 throws a rather important sidelight on the 

 problem. The Mississippi and Ohio are 

 bordered by high rough country and the gen- 

 eral surface slope is not toward these two prin- 

 cipal streams but away from them. The loess, 

 however, here as elsewhere, is thick, porous, cal- 

 careous and fossiliferous on the high hills near 

 the rivers and gradually becomes thin and clayey 

 toward the low country at some distance from 

 them. If the part below 500 feet were water 

 deposited, thousands of square miles of the 

 interior lowland must have been submerged 

 and this area should presumably have a con- 

 siderable deposit if not a thicker one than the 

 higher country, but the deposit is thin or 

 wanting in the lowland. It thickens gradually 

 toward the rivers and is thickest on the hills 

 where the water must have been most shallow, 

 along the Ohio and Mississippi. The bluff 

 loess certainly appears to be a single deposit 

 from the bases of the hills at 375 or 400 feet 

 to their crests at YOO or 800 feet where it is 

 thickest. The distribution of the loess is very 

 different from that of material known to be 

 waterlaid valley filling. 



Briefiy, the writer believes that the so-called 



