86 IOWA ACADEMY OF SCIENCES. 



First. — The vertical distribution of the molluscs. The writer 

 has already shown" that these molluscs were most probably- 

 deposited in situ, and sufficient time must have elapsed at least 

 for the production and developement of the successive genera- 

 tions. 



Second. — The fineness and homogenity of the loess material. 

 This is of import8;nce, for had the deposits been made quickly 

 by powerful concentrated agencies, whether wind or water, 

 much more coarse material would have been mingled with the 

 fine debris. 



Third. — No plant remains of undoubted loess origin occur. 

 As the plants undoubtedly existed during the entire period 

 the deposition must have gone on so slowly that ample time 

 was given the plant remains to crumble in decay and mingle 

 with the soil. 



With these propositions as an aid let us consider the follow- 

 ing conception of the formation of the loess deposits: 



The region formerly covered by the glaciers remained a vast 

 drift-covered plain af ter,tfce recession of the glaciers. 



No loess was to be found, but the surface material consisted 

 of unasserted drift, here and there heaped up in ridges and 

 moraines. Streams soon cut their way through this materiaP- 

 and ponds more or less numerous remained ia the depressions 

 of the plain. 



The climatic conditions having so improved, plants, at first 

 the smaller forms, spread over the plain, and soon trees, in 

 whose shades numerous molluscs lived and prospered, appeared 

 in narrow lines along the streams, the surface conditions being 

 not unlike those of the northwestern portions of the state 

 to-day. Forests gradually spread over portions of the area, 

 principally along the river- valleys and on hillsides in the man- 

 ner pointed out by Prof. Macbride.^-' 



When vegetation, especially the forests, had gained a foot- 

 hold, then commenced the deposition of the loess. 



iiBulJ Kat. HM. S. U. I., Vol. II, p. 95. 



i2lf it be true that our streams generally follow the highest ridges of the drift, even 

 without reference to the loess, i. e. if the streams run in glacial ridges (and the writer 

 knows of some cases where this is true), then the fact can be accounted for by the 

 theory offered in the paper by McGee and Call already cited, pp. 22-23, but the theory 

 fails when applied to the loess because of the climatic conditions required. 



13 See paper: Forest DMrilmtion in loiva and its Sionificaticc, in this volume. 



It is but fair to say that the theories thus presented by Professor Macbride and the 

 writer, while leading to the same results, weie developed from different standpoints 

 .along entirely independent lines of Investigation. 



