GEOLOGY 



2439 



GEOLOGY 



era is inferred because of deposits of carbona- 

 ceous material and of limestones. 



During the Proterozoic Era sedimentation 

 for the first time became the principal process 

 in the formation of rock. There is, however, 

 considerable igneous rock with the sedimen- 

 tary formations. To this era belong the for- 

 mation of the iron ores of the famous Mesaba 

 Range, in Minnesota, and of other Lake Supe- 

 rior regions in Wisconsin and Michigan. These 

 ores were formed for the most part in the 



DC 



UJ _to 



CD' 



y E 



8 

 il 



Lul 



o 



M CL 

 O <y 



Present 



Glacial 



Pliocene 



Miocene 



Oligocene 



Eocene 



Cretaceous 

 Jurassic 

 Triassic 



Permian 



Carboniferous 



N 



E 



Cambrian 



Devonian 



Silurian 



Ordovician 



Keweenawan 



Animikean 



Huronian 



Igneous Rocks 



Granite 



Slate 



Limestones 



ROCK SYSTEMS AND GEOLOGIC ERAS 



Animikean Period (see table). A few fossils 

 have been found in Proterozoic rocks, but 

 these are very rare. However, they make the 

 existence of life in this era a matter of cer- 

 tainty. Other evidences are the presence of 

 carboniferous shales and slates and of lime- 

 stone. 



The Cambrian, the first period of the Paleo- 

 zoic Era, is of special interest and importance 

 because its rocks constitute the oldest system 

 known that contains an abundance of fossils. 

 It is therefore the first satisfactory record of 

 life. This system contains all the common 

 aspects of sedimentary rocks conglomerates, 

 sandstones, shales and limestones but in most 

 regions the sediments have been cemented into 

 solid rock. In some cases the sandstones have 

 been changed into quartz schists, the shales 

 into slates and the limestones into marble. 

 Most of the fossils represent the shells or 

 tracks of marine animals buried in sands and 

 muds, "but the absence of fossils of land species 

 does not prove that no life then existed out- 

 side of the sea. Every great division of the 

 animal kingdom except the vertebrate (back- 

 boned) animals was represented, but even 

 though no vertebrate fossils have been found 

 in the Cambrian series, geologists are not pre- 

 pared to say that there were no vertebrate 

 animals in that period. 



Among the invertebrates known to have ex- 

 isted are several kinds of mollusks, sea worms, 

 sponges, a few forms of the protozoa group 

 (see PROTOZOA) and corals. Traces of soft jelly 

 fish have been preserved. The highest in or- 

 ganization of the Cambrian animals was the 

 trilobite, now extinct. It was related to the 

 modern crab and crayfish and belonged to the 

 arthropoda group (see ARTHROPODA). It is sup- 

 posed that with the abundance of animal life 

 there was a corresponding abundance of vege- 

 table life, to furnish the needed food supply. 

 Geologists also infer from the shells, plates and 

 other forms of protective covering character- 

 istic of so many animals of the Cambrian 

 Period that other species existed of which the 

 fossils bear no record, and against which the 

 known animals had to defend themselves. 



It is not possible here to take up in detail 

 each of the succeeding periods, but certain 

 striking developments may be pointed out. 

 The fossils of the next period, the Ordovician, 

 are almost entirely those of marine inverte- 

 brates, but among them are the first-known 

 remains of fishes. The dominant group of ani- 

 mals is the Cephalopoda (which see), the 



