50 A NATURALIST IN THE GREAT LAKES REGION 



There were great reefs of corals off the shore of the nearby 

 land, and other species of corals that lived solitary lives rather 

 than in colonies were abundant. There were sponges, grap- 

 tolites, polyps, crinoids like stemmed starfish, ordinary star- 

 fish, lamp shells or brachiopods, clams, snails, cephelopods, 

 somewhat related to the pearly nautilus of today, only having 

 long, straight shells instead of coiled ones; there were primitive 

 crustaceans like the trilobites. Undoubtedly on the land about 

 the sea grew giant club mosses, tree ferns, cycads, and the lower 

 types of flowering plants such as the conifers and the grasses. 

 Centipedes and lowly types of insects were also probably seen. 



A period of upheaval followed the deposit of the Niagara 

 limestones closing the Silurian period, and the Chicago region 

 was once more dry land undergoing erosion. How long this 

 condition persisted it is hard to say, but there followed another 

 period of depression when the sea again occupied this site. The 

 inland sea was becoming more restricted, however, deposition 

 was going on in limited basins, and the wash from the land was 

 increasing as the land area grew. In this Devonian period the 

 strata formed consisted of shales and limestones. Only remnants 

 of them exist in a few locations about Chicago, however, for the 

 sea disappeared in our region rather promptly and all the Devo- 

 nian rocks that had formed were eroded away leaving the Niagara 

 limestone bare except where an occasional fissure in it had 

 happened to receive and retain some of the Devonian deposits. 

 Such have been found at the quarry at Elmhurst and at Lyons. 

 These deposits, though scanty, are exceedingly interesting, for 

 they contain many sharks' teeth, local evidence of the presence 

 of the fishes in the Devonian seas; they were so abundant, so 

 dominant, that the Devonian is known as the age of fishes. 



So far as we know, the ocean never again overflowed the site 

 of Chicago. This immediate region remained exposed, subject 

 to the action of the elements, undergoing extensive erosion 

 during the rest of the Paleozoic and during all of the Mesozoic 

 and the Cenozoic. Our local rocks afford no evidence of the 



