32 



GEOIvOGY OF IvA SALLE COUNTY. 



coal beds, which is very clearly shown at Ottawa, and 

 still more so at other places, must be attributed to 

 this cause. 



The distribution of the Trenton is not easily 

 accounted for. It may have been, most unquestionably 

 has been removed over extensive areas, but why at 

 Streator we should have a thickness of 203 feet of this 

 rock and only isolated patches of it in the Illinois val- 

 ley, and on or in the adjacent bluffs, is not easily 

 explained. But such are the facts. It is also some- 

 what puzzling- to find a strip of Trenton limestone ex- 

 tending- nearly across the Illinois valley, about two 

 miles west of the court house at Ottawa, resting- con- 

 formably on the St. Peters, while the north bluff 

 shows St. Peters at a hig-her level, covered by con- 

 formable coal measures and the Trenton wanting-, 

 • and, to make the puzzle g-reater, Buffalo Rock, a 

 vast, isolated mass of St. Peters, capped with coal 

 measures, rises some sixty-five feet hig'her, and is but 

 a mile farther west. But the difficult}^ vanishes if we 

 suppose the eastern slope of the ridg-e to be near 

 the- w^est side of the Trenton area, and it is possible 

 the latter may cross the low land north of the C, R. 

 L & P. tracks, and may be found near the bluff in that 

 direction. We have, at least, a curioush^ warped sur- 

 face for the eastern boundary of the ridg-e, which then 

 becomes a low plateau with its crest near the western 

 border. 



The limestones of the calciferous are not pure 

 carbonate of lime, or lime and mag-nesia, but contjiin a 

 considerable percentag-e of alumina or clay. An ideal 

 combination is lime, 56 parts, alumina, 36 parts, and 

 oxide of iron, 8 parts, or 61 parts lime to 39 parts 

 alumina. Specimens from Rondout, N. Y., g-ive car- 

 bonic acid 34.2, lime, 25.5, mag-nesia, 12. 35, silica, 



