132 T. E. Savage — Stratigraphy of Southwestern Illinois. 



imity to Ozarkia this basin was subjected to vertical move-, 

 ments and therefore to variable conditions of sedimentation, very 

 different from those that prevailed during the same time over 

 the more northern areas. 



Ordovician. 



Galena-Trenton. — A thickness of 68 to 80 feet of this for- 

 mation is exposed in Alexander county. It appears at two 

 points adjacent to the Mississippi river where the waters of 

 that stream have cut across low arches which bring the Galena 

 limestone above the level of the water. One of these expo- 

 sures is a short distance below Thebes, where a thickness of 

 about 6S feet of limestone may be studied. The second fold 

 crosses the river about two miles north of Thebes, just west of 

 the village of Gale, where these limestones may again be seen 

 on Little Rock Island. 



The Galena formation is here a light colored, crystalline, 

 non-magnesian limestone, in layers from a few inches to four 

 feet in thickness, which are imperfectly exposed in the upper 

 part. The lowest layers contain in abundance Receplaculites 

 oiveni, Hebertella near occidentalism Parastrophiahemiplicata, 

 Platystrophia biforata, Rafinesquina alternata, Rhyncho- 

 trema inwquivalve, Strophomena emaciata^ Triplecia n. sp., 

 and the trilobites Bronteus lunatus, Rumastus trentonensis, 

 lllcenus americanus, Isotelus maximus and Platymetopus 

 cucidlus. Eighteen feet above low water Crania trentonensis, 

 Cyrtolites ornatus, Plectorthis plicatella and Remopleurites 

 striatulus are associated with most of the above mentioned 

 forms. In the middle and upper parts the white color is in 

 places mottled with pink, and the fossils become much less 

 abundant. Receptaculites oiveni is still common, while Crania 

 trentonensis, Hebertella near occidentalism Platy atrophia bifo- 

 rata, Rafinesquina altemata, Rhynchotrema incequivalve 

 and Triplecia n. sp., persist in diminished numbers. This 

 facies of the Galena resembles, in its fossils and lithology, the 

 Kimmswick limestone of Ulrich, also described by Weller 

 from Jersey and Calhoun counties.* The basin in which it 

 was deposited may have been somewhat separated from that 

 which received the sediments of the more northern dolomite 

 phase of the Galena. 



Richniond-Maquolceta. — The beds provisionally referred to 

 the Richmond have an aggregate thickness of 91 feet. This 

 formation succeeded that of the Galena after a long land 

 interval. All of the Utica and Lorraine deposits are wanting, 

 and, seemingly, much of the Richmond is also absent. The 

 formation in southwest Illinois consists of two members, 2a 

 * Weller, Illinois State Geological Survey, Bull. No. 4, p. 222. 



