MIDDLE AND UPPER ORDOVICIAN. 169 



equivalent may occur in any of the ranges where the Cambrian and Ordovician are 

 mapped together or where the Silurian is shown, throughout Nevada and western 

 Utah. 



Ulrich " regards part of the Lone Mountain limestone as much older than the 

 Bighorn dolomite and Whitewood limestone of Wyoming or the Harding sandstone 

 and Fremont limestone of Colorado. According to Kindle *^* the beds which have 

 been classed as Silurian, on account of containing Halysites catenulatus, may in 

 general be regarded as possibly Ordovician, and Ulrich states that Halysites occurs 

 generally in the West in faunas which are regarded as of Richmond age but is also 

 known in faunas which are correlated with late Black River horizons. 



J 13. SOT7THEBN COLOBADO AND NEW MEXICO. 



Throughout much of southern Colorado, New Mexico, and Arizona Ordovician 

 rocks are generally lacking and the hiatus corresponding with the period also com- 

 prises earher and later epochs, down to the pre-Cambrian and up to the Carbonifer- 

 ous, over extensive areas. Ordovician deposits along the shores of the Colorado 

 land constitute the Harding sandstone and related formations. At Canon City, the 

 type locality,\the sandstone rests on Algonkian strata, is 86 feet thick, and is over- 

 lain by the Fremont limestone, 200 feet thick. The sandstone contains the oldest 

 known fish remains^^^* and the limestone an invertebrate fauna containing Trenton 

 and Richmond fossils." The Fremont limestone is overlain by Carboniferous lime- 

 stone, Silurian and Devonian strata being absent, yet, as stated by Crosby,^*' the 

 line of demarcation "is not strongly defined, though it represents a long period of 

 nondeposition and a great time break." 



J 15. NOBTHEBN ABKANSAS. 



The Ordovician section of northern Arkansas is very incomplete. There are 

 unconformities at several horizons and the formations are irregular in thickness to 

 the extent of being entirely lacking in some sections though present elsewhere. 

 Ulrich *^" states that the equivalents of the upper Stones River, Black River, Tren- 

 ton, and Utica are absent. The lower Stones River and the Joachim dolomite of 

 Missouri are represented by the Izard limestone, the lower Richmond by the Polk 

 Bayou limestone, and the upper Richmond by the Cason shale. Beneath the Izard 

 limestone there is in many places a representative of the St. Peter sandstone. 



J 15-16. MISSOTJBI AND SOXPTHEBN ILLINOIS. 



The lower Ordovician of Missouri and southern Illinois comprises the later 

 formations of the "Magnesian Limestone series" or "Ozark series." The latest 

 published classification is given in Chapter III (p. 104), according to Ulrich. 



The strata placed in middle and later Ordovician comprise the Trenton, Hudson, 

 and Cape Girardeau of Keyes's section.*"^ More recent discussions bearing on the 

 correlation of the "Magnesian Limestone series" and the St. Peter sandstone have 

 been published by Hall and Sardeson ^°^ jointly and by Berkey.'^^ The strata to. 

 which Keyes applied the name Trenton are now known as Plattin Kmestone (at the 



o Ulrich, E. 0., personal note. 



