358 W. J. McGee — Geology of the Hfississipjri Valley. 



tities of chert, or hornstone, both in perfect nodules and in 

 splintered fragments, mingled with the drift. 



The average thickness of this member may be roughly stated at 

 20 or 25 feet, though it is very variable. Its base is far from being 

 horizontal, but is to a remarkable degree conformable with its 

 surface ; and the same may be said of the subjacent member. 

 Before the deposit of either, the surface of the sedimentary rocks 

 had been eroded into valleys and ravines, and a like process took 

 place subsequent to the deposit of each ; and the recent contours 

 correspond closely with their pre-glacial prototypes. 



Member number three is by far the most continuous and constant 

 over wide areas of any of the superficial deposits. It is only inter- 

 rupted by occasional ridges and roches rnoutonnees where the older 

 rocks are exposed, and in a few valleys where removed by erosion. 

 It does not extend as far southward as the subjacent member, 

 however. On the meridian of 89° W. it was only traced to about 

 lat. 89° 30', while the blue clay extends nearly or quite a degree 

 further. Along its southerly margin it gives place to a deposit 

 usually spoken of as littoral or lacustral (but which is probably 

 only a considerably modified glacial drift) ; and only the underlying 

 formation is usually considered to be of northern origin. Further 

 to the northward, where number three is well developed, it is 

 universally recognized as glacial drift, but is seldom distinguished 

 from the next member. The writer has traced this so-called 

 lacustral deposit of the southern drift regions northward into the 

 region of member number three, and found the one to graduate 

 into the other ; the relative position of the next member mean- 

 while remaining constant. 



Member number four. — The colour of this member varies from 

 bluish white to black ; the prevailing colour, where not mentioned, 

 being deep blue. This colour is due to the presence of black 

 oxide of iron. In North-eastern Iowa it rarely contains boulders or 

 pebbles, though in other localities they have been found quite 

 abundant. Such as are found in that region are usually of the 

 hornblendic rocks, syenite, diorite, hornblende schist, etc. Granite, 

 which characterizes the superincumbent member, is rarely seen in it. 

 This distinction has not been observed in other localities, and is 

 probably only local. A much larger proportion of the boulders is 

 striated than in member number three. The pebbles are both local 

 and erratic, and there are sometimes intermingled fragments of 

 ferruginous conglomerate and lumps of dark brown granular clay, 

 easily identified with member number six. The materials are some- 

 times imperfectly stratified, having intercalated beds and bands of 

 sand, gravel, or differently coloured clay, often lenticular in form. 

 Frequently the clay is very fine and clean, affording an excellent pipe 

 or potter's clay, and exhibiting a columnar and jointed structure 

 when dry. Parts of it are sometimes so indurated and laminated as 

 to resemble shale in structure and consistency ; and fragments of 

 this shale are occasionally so highly bituminous as to be imperfectly 

 combustible. This is most frequently the case where quantities of 



