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GEOIvOGY OP IvA SALLE: COUNTY. 



In the lan^uaofe of g-eolog-y all of the material of 

 the g-lobe is termed rock, whether hard and unyielding- 

 like g-ranite and limestone, or soft and pliable like clay, 

 or loose and incoherent like sand. All rocks are 

 divided into two g'reat classes — stratified, arrang-ed in 

 layers, strata or beds; and unstratified, showing" no 

 layers or beds. Another division is into ig^neous, the 

 result of volcanic action, and sedimentary, deposited by 

 water. The sedimentary are divided into fossiliferous, 

 those containing- more or less of animal and veg-etable 

 remains, and nonfossiliferous, containing- no remains of 

 plants or animals. Sedimentary rocks are, of course, 

 stratified, and all fossiliferous rocks belong- to that 

 class, but all stratified rocks are not fossiliferous. 



The rocks of the Central Plain are, with few 

 and not important exceptions, stratified, and most 

 of them more or less fossiliferous. 



While larg-e bodies of unstratified rocks have been 

 produced by volcanic action, others are evidently only 

 sedimentary beds chang-ed by heat and pressure, so 

 that they have assumed a new form, for we may in 

 some cases trace stratified beds to where they lose all 

 evidence of stratification. It is now believed that 

 by far the larg-est part of all known rocks is of 

 sedimentary orig-in. Rocks differ much in composition, 

 character, color, hardness, weig-ht, etc. According- to 

 composition they may be divided into calcareous or 

 lime rocks, arenaceous or sand rock and arg-illaceous or 

 clay rocks. A rock is often made up of two or more 

 kinds of material. Hence we have calcareo arena- 

 ceous, calcareo-arg-illaceous, areno-arg-illaceous, etc., 

 rocks. In general, unless there are special reasons for 

 a more careful desig-nation of character, we shall call 

 those rocks of which sand, thoug-h not the only, is the 

 principal component, arenaceous; those of which lime 



