Fossil Nautiloid 47 THIRD FLOOR EXHIBITS trapped by the alg ae and a few open pore spaces The snail shell is flled with lim y mudstone calcite and the fossiliz ed shell of a smaller snail Gift from Malcolm P W eiss Northern Illinois Univ ersity DeKalb Impact Br eccia Breccias lik e this one from the Onaping F ormation o verlie the ric h ore bodies of the Sudbury mining district in L evac k Ontario Canada They were once thought to be v olcanic breccias but they are now recogniz ed as deposits formed by the catastrophic im pact of an asteroidsiz ed body almost 2 billion year s ago F ragments of roc ks blasted out by the impact fell bac k into the crater and were altered and cemented by upwelling magma Gift from Be v an M F rench Goddard Space Flight Center Greenbelt Maryland V olcanic Br eccia The roc k fragmentsor clastswithin this specimen are v olcanic in origin Their angular edges indicate that they were transpor ted a limited distance af ter they were ejected by the v olcano This slab from the S an J uan region of southwestern Colorado has been polished to highlight the fragments Gift from Edwin Eck el US Geological Surv e y Denv er Colorado and for mer e x ecutiv e secretary of the Society F ossil Nautiloid These extinct creatures are of Dev onian age and li ved in a shallow sea about 350 million year s old where the Atlas Mountains of Morocco are now They are distinguished by a straight rather than curved shell with a central si phuncle and septa curved concave tow ard the li ving c hamber This polished fossil is a relati ve of today s c hambered nautilus Presented to GSA as a centennial birthda y gift b y the American Association of P etroleum Geologists through F red A Dix Dinosaur F ootpr int This is one of three dinosaur trac k specimens at GS A from the Connecticut Ri ver V alley in Massac huset ts the other two specimens appear on the w all abo ve the stairwell on the other side of the third foor F or more information see p 53 P er manent loan from the Pratt Museum Amherst Col lege Amherst Massachusetts through Gerald P Broph y Silicifed W ood with Quar tz Crystal Lik e most petrifed wood this specimen is pres e r v e d primarily by c halcedon y a form of quar tz with crystals too small to see However indi vidual quar tz crystals are visible on some of the petrifed wood s surfaces V isible crystals lik e these usually form in open spaces or hollows that existed in the wood when it w as buried by sediment