10 



GEOLOGY OF LA SALLE COUNTY. 



suited to the needs of the being-s, whose tomb these 

 rocks are. We are sometimes astonished at the rapid 

 thickening' or sudden thinning- out of a particular layer, 

 but we must remember that then, as now, the shore line 

 ^Yas not always the same, the land oscillating-, sinking- 

 here, rising- there, so that formations would be short- 

 ened in one direction and lengthenel in another. Thus 

 let aa, plate 2, fig-. 9, be the level of sea, bb shore, 

 cc a la^-er of sediment. Then if the land sink so that 

 ee is the level of the sea, the sediments will extend to 

 that heig-hth, and the beds at all points below aa be 

 much thickened. 



Chang-es of level are not usu'^lly rapid, as far as 

 we know^ except in a f ew^ cases, a foot to three feet in 

 a centur}^ but there may be periods in our planet's his- 

 tory when the forces causing- these curious phenomena 

 are more active than at others, and when there ma}^ be 

 in certain areas a rapid rise or settling- of the land, and 

 a consequent change in the area covered by water. We 

 must not forg-et that all our strata were deposited 

 in water, that the dr}^ land is not built upon, and that 

 the remains of its inhabitants are not preserved except 

 the bones, and these rarely, unless they happen to be 

 swept into the ocean or a swamp wh^re they happen to 

 be covered by mud, and thus sealed up in a casket 

 of nature's own manufacture and embalmed in a man- 

 ner that mocks at art's efforts to imitate it. 



The rocks are but tlie catacombs, hoar}' with the 

 dust of ag-es, from which the g-eolog-ist patient 

 research bring-s forth the mummies of races that per- 

 ished, and whose tombs were g'ray with ag-e ere man 

 had laid the first stone of the first pyramid, or the 

 tribes that lived between the rivers had made the first 

 brick for the most ancient of Nineveh's palaces. These 

 are truly Nature's medals cf creation, a mig-ht}' 



