GKOLOGY OF LA SALLE COUNTY. 



17 



reddish, reddish brown, bluish and silvery white, and 

 in hardness from a very soft pulverulent mass to very 

 hard rock. Much of it is composed of almost perfect 

 quartz cr^^stals, clear as tlie clearest w^ater, and with- 

 out speck or flaw. While the planes of stratification 

 are but faintly marked, the rock is often laminated, 

 and the planes of lamination are as distinct as are the 

 former, and may, unless carefully observed, be mis- 

 taken for them. Plate II, fig-. 10, aa, are planes of 

 stratification, bb the planes of lamination which vary in 

 inclination not only in the diflFerent beds, but in differ- 

 ent parts of the same bed, and are sometimes wholly 

 w^anting-. 



The St. Peters, as far as w^e have examined it, is 

 utterly destitute of fossils, or any traces of either ani- 

 mal or veg-etable remains, and is in g-eneral character 

 one of the most homogfenous of rocks. It forms the 

 floor of the Illinois Valley from Ottawa to a point 

 about a mile east of Utica, the north bluff from a mile 

 west of Ottaw^a to Utica, and the south bluff from a 

 point four miles west of Ottawa to Little Rock. It 

 also forms the bluffs of Fox river to beyond Sheridan, 

 and ag-ain appears in the bluffs near Milling-ton, and to 

 the northeast of that place. It is the rock into which 

 all the deep ravines opening into the Illinois valley rea 

 cut from one-fourth to one-half mile before reaching" 

 the valley. On the north side of the river, in the 

 ravine of Clark's Run, it is exposed for more than a 

 mile from the bluff. It is g-enerally inclined to the 

 south and southw^est, the slope being* very moderate, 

 but at Milling-ton it appears in g-reat arch-like rolls as 

 shown by plate II, fig*. 11. While along- the Big- Ver- 

 million, it is often inclined at an ang-le of 45 ^ , as is 

 alst the case at the tunnel, near the La Salle-Utica 

 line. Some of the strata are, in places, rich in bisul- 



