360 W. J. McGee — Geology of the Mississippi Valley. 



combine with the oxygen of the air. Even to-day the clay is some- 

 times so fully charged with the gases of organic decomposition that 

 foul smells are emitted for years into wells penetrating it ; and in not 

 a few instances the water has been rendered wholly unfit for potable 

 or culinary uses. In several cases, indeed, it has been suspected 

 that sickness was due to the use of water collected from this member. 



Ferruginous concretions are common in this member, and thin 

 layers of bog-ore are occasionally found. Cylindrical concretions of 

 clay, stained and partially cemented by sesquioxide of iron, frequently 

 occur. They exhibit a concentric structure when viewed in cross- 

 section, and are gnarled and knotted like twigs and rootlets. They 

 may be hollow, or may yet contain the remains of the woody 

 substances around which they were originally formed. 



Member number four is believed to be the equivalent of the 

 Lower Till of the British geologists, and probably of the Canadian 

 Erie clay of Sir William Logan. It is observed to the best advan- 

 tage near its southern limits. Further to the northward the later 

 glacier has sometimes removed it entirely, and often mingled its 

 materials in inextricable confusion. Still, its distinctness from the 

 later drift is nearly always very strongly marked. In one instance 

 only have their positions been observed to be reversed, and this in a 

 valley (of the Illinois Eiver, at Aurora, Illinois), and very probably 

 by post-glacial agencies. Where found, its thickness usually 

 averages about half that of the whole series of surface deposits ; but 

 it is absent on rocky divides, and sometimes in valleys of erosion. 

 In view of their extensive commingling, and sometimes reversal of 

 position, it is scarcely deemed expedient to distinguish it from the 

 forest bed recognized by Dr. Newberry in Ohio, and by both British 

 and Continental geologists on the other (European) side of the 

 Atlantic. 



Member number five. — This formation is usually thin, generally 

 ranging from one to five or six feet, but occasionally expanding to 

 a much greater magnitude. Its principal constituents are coarse 

 sand, gravel, pebbles and small boulders, and coarse clay ; and either 

 may predominate. The character of the pebbles is the same as in 

 the overlying member, except that there is a larger proportion of 

 local materials. Ground and striated pebbles are more frequently 

 found in this than in any other member. 



This member almost invariably accompanies the last, and usually 

 rests on the older rocks. It is of considerable economic importance, 

 as, within the frequently stratified and laminated sand and gravel 

 composing it, an abundance of good water is usually found. It 

 sometimes graduates into the overlying member. No well-marked 

 distinction between the two can be observed, except that of colour — 

 the lower having generally a dirty yellow or yellowish brown 

 colour. If the colouration of the upper member is due to the 

 agencies suggested, it is a little difficult to see why the process 

 should have stopped on reaching the more pebbly and stratified clays. 



Member number six. — This basal member is very unfavourably 

 situated for observation, and is so often absent that its occur- 



