18 



GEOLOGY OF LA SALLE COUNTY. 



phide of iron (pyrite), and much harder than the adja- 

 cent beds, but the decomposition of the pyrite leads to 

 the rapid disinteg-ration of these hard beds, and they 

 g-ive way before the attacks of moisture and frost 

 almost as rapidly, sometimes faster, than the softer and 

 apparently less stable rocks. 



In some places this rock forms perpendicular cliffs 

 90 to 130 feet hig-h; in others the lower strata have 

 decayed, leaving* the upper overhang-ing* for many feet 

 — 70 to 80- -and forming- what are locally called caves, 

 a noted one of which is Atwo(.d's, Clark's or Del- 

 bridg-e's cave, about seven and a half miles southwest 

 of Ottawa, on the road to Deer Park. Others quite 

 as fine exist along* the Illinois at various points, espe- 

 cially between the old slaug-hter house and Ma3^o's 

 ravine and ferry on the south bank of the river. The 

 canons also furnish several fine examples of such 

 recesses or rock shelters, some of which would furnish 

 very comfortable quarters for twenty-five or thirty 

 persons. See plate II, Pig-s. 1, 2, 3. The Trenton is 

 overlaid conformably, as far as is known, by the car- 

 boniferous or coal measures, a g*reat collection of clays 

 clay shales, sandstones and limestones- There seems 

 to have some time intervened between the close of the 

 St. Peters era and the beg^inning* of the carboniferous, 

 for the surface of the former, where uncovered by the 

 removal of the coal and clays, is seldom smooth, but 

 g-enerally pitted with bowl-shaped cavities and dome- 

 shaped hillocks, with here and there a cut as if it had 

 been the bed of a stream mentioned above seven or 

 eig-ht feet deep, and the time, g-eolog'ically speaking-, 

 may not have been very long*. 



At Ottawa, on the bottom east of the Fox river, 

 as in Fig*. 1: Thin bedded clays, 7 to 8 feet; hard clay, 

 6 to 7 feet; coal, 22 inch; gypsum, 1 to 1>^ inch; fire 

 clay, 3 to 10 feet; St. Peters sandstone. 



