14 INTRODUCTION. 



none whatever in Iowa except the granite bonlders so com- 

 monly seen in the drift which almost everywhere covers the 

 snrface of the State like a mantle, and which forms the soil 

 and sub-soil over much the greater part of Iowa. Therefore 

 all the rocks of Iowa except these granite strangers are strati- 

 fied rocks. 



All such rocks have received their stratified form by having 

 been originally deposited as a precipitate or sediment in 

 water, and with rare exceptions they have been deposited in 

 the waters of the sea when it occupied that portion of the 

 surface of the earth where they are now found, and which has 

 since become dry land by the gradual elevation of the conti- 

 nent or island mass, as the case may be, above the level of 

 the sea. In other words, the majority of stratified rocks are 

 of marine origin, the sediment subsequent to its deposition 

 having become more or less hardened. All the rock forma- 

 tions of Iowa are stratified, and all are of marine origin, 

 having been deposited in seas that gradually and successively 

 receded to the southward and westward, as the continent was 

 evolved, leaving their own consolidated and stratified sedi- 

 ment to form the foundations of what is now the State of 

 Iowa. 



The periodical changes in the character of these successive 

 deposits, together with the characteristic remains of life that 

 existed during the slow deposition of each, now afford us 

 the material, by the classification of which we are able to 

 construct a consecutive history, not only of the order in which 

 the different rock formations were deposited, but also of the 

 order in which the forms of life themselves were successively 

 introduced and which characterized each epoch of time that 

 each formation represents. These formations are composed 

 of assemblages of strata which have characteristics in com- 

 mon and extend over very large areas like broad sheets as it 

 were, lying the one above the other, the edges of each receding 

 the one from the other at a greater or less angle of dip, so 

 that although the greater part of every formation is out of 

 reach beneath others, we have the opportunity at some point 



