215 



Ft. In, 

 16 — Shale, shaly limestone, some thick layers of limestone. Rafine>i- 



quina, Bryozoa, Trilobites, Zygoxpira (aaa) in some layers 40 



J 5 — Heavy layer of limestone & 



14 — Shale and shaly limestone. Trilobites (a^ 9 6 



13 — Limestone. liajinesquina, AsaphuK, Cali/mene, Plaiyslrophia 8 6 



i2— Mostly shale 4 10 



U — Argillaceous compact layer. Trilobites, Gastropods 6 



10 — Shaly limestone ' 2 9 



9— Limestone and shale. Plalystrnphia. Hebertella 3 



8 — Limestone. P. lynx and laticosta. Hebertella 3 6- 



7 — Covered 10 



() — Limestone, more shaly than 5. Hebertella (form with dorsal fold), 



P. lynx and laticosta 3 



5 — Same as 4 but P. laticosta more abundant 3- 10 



4 — Thin shaly limestone. Hebertella 1 2 



3 — Thin shaly limestone. Hebertella, P. laticosta, 3Ionticulipcra (aa) . 1 

 2 — Thin shaly limestone. P. lynx (aa), laticosta (a), Hebertella occi- 



dentalis (c) Trilobites, Monticulipora 1 



1 — Covered to river level 45 



Total section 198 



This section shows some 30 feet more of the Platystrophia beds than the 

 Madison section ; otherwise it is the same in the main as the basal part of that 

 section. 



P, lynx is here abundant in the lower layers and laticosta in the upper layers. 



From the detailed sections now described of the Ordovician rocks of Indiana 

 it will at once be seen that there are certain well-defined faunal zones which may 

 be traced without fail over the whole area. It has long been known that the 

 zone characterized by the presence in abundance of Platystrophia, forms a well- 

 marked and persistent stratum ; but apparently the other zones, if reco<'-nized at 

 all, have been minimized in importance. Any one of them is however as per- 

 sistent and as easily traced as the Platystrophia zone. 



These zones are in ascending order as follows (the thickness is given in pa- 

 rentheses) : 



1— Dalmanella multisecta (200-240 feet). 



2 — Piafinesquina allernata 50-70 feet). 



3— Platystrophia (60-80 feet). 



4 — Rafinesquina alternata var. fracta (100 feet zh), 



5— Dalmanella Meeki (20 feet ±). 



6 — Streptelasma. 



7— Strophomena (10 feet ±). 



8— Rhynchotrema capax (10 feet zb). 



