4 



GKOIvOGY OP" LA SAIvIvE COUNTY. 



An inspection of this table shows that the g-eo- 

 logical series is not the same in all places, that some 

 members are often wanting-, and that the same beds 

 vary g-reatly in thickness at points but a few miles 

 apart, and that strata may utterly disappear in the 

 course of a few miles. There is probably no place in 

 the w^hole world where all the formations are found, 

 and but few w^here any one of the eras is completely 

 represented. 



The ofreatest thickness of the stratified rocks is 

 variously estimated, for they nowhere are so exposed 

 as to admit of direct measurement, and even where 

 measurements are possible they are usually inclined, so 

 that it is not the actual thickness that we measure, but 

 a diaofonal to it — the hypothenuse of a rig'ht-angled 

 triang-le, the heig'ht of which is the distance we seek. 

 These calculations give a g-reatest thickness for the 

 sedimentary rocks of not less than 120,000 feet, or 

 more than 22 3-5 miles, and a least thici^ness of eig-ht 

 to nine miles. 



We observe from the above table that many entire 

 eras are not represented in Illinois, and that some 

 of which portions are found in it do not appear in La 

 Salle county, that really from the carboniferous to the 

 g-lacial epoch there was little or no addition to the 

 strata of this part of the earth. This means that it 

 was no long-er a sea shore reg-ion or an area of shallow 

 lag-oons or lakes, but dry land. Chang-es there un- 

 doubtedly were, it may be g*reat ones, but they did not 

 result in building- up g-reat accumulations of sediments 

 in'which were entombed the remains of the strang-e 

 inhabitants of those lands. For long- ag-es the sun 

 looked down on, it may be, a g-reat forest reg-ion or 

 g-rassy plains, over which roamed strang-e, wierd forms, 

 t) our minds uncouth and frig^htful, the then lore's 



