168 GENERAL GEOLOGY. 



the map-model in another part of this volnme, bnt its position 

 is shown on the larger geological map. The exposures in 

 Iowa are found principally upon the banks of the Big Sioux 

 river, for which reason the specific name of Sioux Quartzite 

 has been given to it. 



It is an intensely hard rock, breaking with a splintery 

 fracture, and a color varying in different localities from a 

 bright to deep red. Although it is so compact and hard the 

 grains of sand of which it was originally composed are yet 

 distinctly to be seen, and even the ripple marks upon its 

 bedding surfaces are sometimes found as distinct as they 

 were when the rock was a mass of incoherent sand in the 

 shallow waters in which it was accumulated. The lines of 

 stratification are also quite distinct, but they are not usually 

 sufficiently definite to cause the mass to divide into numerous 

 layers. It has, however, a great tendency to break up by 

 vertical cracks and fissures into small angular blocks. The 

 process of metamorphism has been so complete throughout 

 the whole formation that the rock is almost everywhere of 

 uniform texture, and its color also being so nearly uniform 

 there is no difficulty in identifying it wherever it may be seen. 



The exposures of this rock, at the localities named, occupy 

 the surface over a number of acres. It also forms some bold 

 fronts by the side of the stream, and causes a slight fall 

 in the river just where the northern boundary line of the 

 State meets it. The dip is distinctly perceptible to the eye, 

 being some four or five degrees to the northward, and the 

 trend of the outcrop is eastward and westward. Although 

 no other kind of rocks, in place, are seen within many miles 

 of this locality, there is little doubt that the friable rocks 

 of Cretaceous age, which appear farther down the Big Sioux 

 in the bluffs which border its valley there, really rest uncon- 

 formably upon the quartzite near where the latter rock 

 appears at the surface. If the Cretaceous rocks do really 

 exist in the uplands of the extreme northwestern part of 

 the State as they seem to do, we have the interesting fact 

 of the newest stratified rocks of the State resting directly 



