OllICIN OF THE SAND8TONKS. 187 



more efVootually tlie percolation of underLii'ound waters and tlieir solvent 

 power than the more coarsely crystalline ))ortions associated with them. 



The Genesis of the Series. 

 the saxdstoaes. 



Silicitms sandstones are heing deposited at the })resent time as marine 

 iiecumulations and river dehris; yet nowhere, so far as the writers can 

 read, are the}' so white and mono-component as are the lower Paleozoic 

 sandstones of the upi»er Mississippi valley. Throuijhout most of the heds 

 tliere is only one constituent, and that is clear quartz. As has already 

 been shown, there are im}Hirities, the chief two being an extremely fine 

 kaolinic material and a ferric oxide, which colors the beds locally. 

 Nowhere have been found those other minerals, rutile, zircon, etcetera, so 

 frequent* in common sands and sandstones. 



To assume that the conditions of sandstone-building in the early 

 Paleozoic were fundamentally different from those of today offends the 

 judgment and soberer second thought. The sources of supply for the vast 

 quantities of quartz which these sandstone beds display was doubtless 

 in the extensive mountain ranges which then must have stood to the 

 north and west, a few of whose ridges still stand exposed as isolated 

 stumps of schistose, gneissic and granitic masses in the outcrops of the 

 region between lake Superior and the Black hills, or are reached in other 

 localities by the tool of tlie well-borer. 



The extent of those ranges cannot here l)e discussed, neither can the 

 mode of deposition of the sediments derived from their degradation. 

 The latter, however, may be assumed to have l)een laid down along a 

 vast shore-expanse, perhaps a Cambrian continental plateau in area, 

 scarcely less than the plateau off ea.stcrn North America at the present 

 time, since the deposits stretch from eastern Wisconsin through Miime- 

 sotii, Iowa and Missouri. Further, the conditions of deposition must 

 luive been extremely unifnrm and (juiet. Periods of deeper submerg- 

 ence are marked off in the continuous history by l)eds of dolomites — the 

 remains of original limestone deposits. 



The origin of the sandstones may confidently be referred to the erosion 

 of crystalline rocks, consisting of granites, gneisses, schists, and (juartz- 

 itcs, ])ecause of the uniformly pure silica which makes up the successive 

 beds. The pebbles in any sandstone are characteristic of the series from 

 wliich they are derived, so basi(^ cruptives could have ))layed no ])art in 

 the production of these clean and almost wliolly silicious beds.f 



As they were originally formed, these sandstone beds were more or less 



• A. Golkio : A T«xt-l.ook of (ioo\ony, third e«nilon. I.ornJon. 1R9.1, p. 120. 



fCompftn* IrviiiKand Van Hi«o: Tim Ponokoo Iroti-buiiriiiK Sorlon of .MiclilKuii uiul WiscoiiHin. 

 Mon. xix, U. S. ticol. Survey, 1802, p. 402. 



