W. J. McGee — Geology of the Mississippi Valley. 361 



rence is the exception rather than the rule. It seldom exceeds 

 three or four feet in thickness, and often only fills the crevices in 

 the rocks. It is rarely stratified, but often exhibits false bedding 

 and laminatiou in lines conformable with its irregular base. It 

 usually consists of a deep brown clay, quite hard when wet, and 

 resisting erosion very perfectly, but crumbling into minute angular 

 fragments when dried. When a freshly exposed lump of it is 

 broken, the surfaces are found to have a granular or crystalline 

 appearance, like that of granite. This structure renders the lumps 

 of it found in the blue clay easily recognizable. Though the colour 

 of the greater part of it is due to the presence of the hydrous 

 sesquioxide of iron, it is sometimes interspersed with specks and 

 streaks of a light yellow colour. 



It is sometimes sandy, pebbly, and gravelly, and its pebbles are 

 both local and northern, rarely striated. When sandy or gravelly, 

 the materials have been generally cemented into a puddingstone by 

 means of the infiltration of ferriferous solutions and the deposit of 

 limonite. Fragments of this conglomerate are found scattered 

 through all the upper members. The pebbles in the conglomerate, 

 as well as those in the clay, where not cemented, are usually stained 

 externally, and for a little distance beneath the surface, by ferric 

 oxides. Good exposures of this member are very rare. Consider- 

 able quantities of the conglomerate (these of very fine materials) 

 are seen at Eockville, Iowa. With a little experience and some 

 care this member is readily distinguishable from the superimposed 

 deposit ; and the one never graduates into the other. 



Disintegrated Materials. — None of these formations should be 

 confounded with a fundamentally distinct one, due to the secular 

 disintegration and chemical decomposition of the sedimentary or 

 crystalline rocks by atmospheric agencies. The importance of these 

 processes are strongly insisted upon by Prof. Eaphael Pumpelly ; x 

 and the occurrence of such a formation over a great part of the 

 Level Region of Wisconsin has been pointed out by Prof. J. D. 

 Whitney. 2 Accumulations of this character, often 20 or 30 feet 

 thick, are well known to the writer as occurring over the Niagara 

 Limestone along the western border of the Wisconsin " driftless 

 region." They are distinguished by great numbers of nodules of 

 chert, often fractured by internal expansion, in the positions in 

 which they were formed in the rock, by the presence of siliceous 

 fossils, mainly corals and crinoid stems, with a few bivalves, and by 

 the absence of erratics. Two miles north of Farley, Iowa, such a 

 bed reaches a thickness of 25 feet, and is overlain by true glacial 

 drift. It bears no evidence of having been disturbed since the 

 decomposition of the rock. 3 



1 In a paper read before the National Academy of Science, April 10th, 1878. 

 See also Am. Journ. Sci., iii., vol. xvii., Feb. 1879, p. 135, et seq. 



2 " Geology of "Wisconsin," 1853, vol. i. p. 121. 



3 This is important as bearing on the question whether a Continental glacier 

 removes all of the lighter materials, and lays bare the rocks over which it passes. 



[To be concluded in our next Number.) 



