CrEOLOGY OF LA SALIvE COUNTY. 



5 



of creation, the masters of a world that the most pow- 

 erful imag-ination can scarcely picture to the mind. 

 We have used the words time, ag-e, period, epoch, 

 as the names of divisions of the g-eolog-ical scale; a 

 word as to their meaning-. A time is a division 

 embracing- aU those rocks, the fossils of which have a 

 common character in their relation to the life of the 

 present. An era is a division of a time, the fossils 

 of which have many features in common. A period is 

 a smaller g-roup of strata of which the fossils have a 

 very close similarity, and an epoch is a division distin- 

 g-uished from others by the presence of certain fossils 

 not found in other beds. If we surve}^ the strata of 

 the United States we shall find that the Cambrian is 

 well represented about Lake Superior and in Northern 

 New York; the Silurian in Central and Western New 

 York and along- the Appalachian Mountains; the Car- 

 boniferous on both sides of the Appalachians, but 

 especially on the western side; the Triassic, Jurassic, 

 Cretaceous and Tertiary along- the Atlantic and Gulf 

 coasts and about the Rocky Mountains; and the Glacial 

 in the northeastern and northern part of the country, 

 almost entirely confined to the section north of the 

 Ohio and Missouri rivers. 



By strata we mean beds or layers. It is very 

 probable that these strata were orig-inally nearly hori- 

 zontal — there were, no doubt, slig-ht slopes, it may be, 

 of 10 ^ to 12 ^ — but there could have been no very 

 steep inclines. As all sedimentary beds were deposited 

 from water they were laid down on the sea shore or in 

 lakes. As a consequence, we ma\^ infer that the area 

 over which any beds may be wanting- was at the time 

 they were being- formed dry land, unless their absence 

 can be satisfactorily accounted for in some other way. 

 The strata are not usually found in a horizontal posi- 



