CAMBRIAN AND LOWER ORDOVICIAN. 103 



Above this the strata are often wavy in bedding, witli shale partings, in which mud cracks, 

 trails, and other markings are common. These layers are friable sandstones in places. More 

 massive quartzite layers generally succeed this series, but not in a thickness equal to those 

 below. 



At the very base of the formation a true basal conglomerate is often found but only in 

 hollows of the granite, schist, or Algonkian quartzite floor. The hard Algonkian quartzites 

 are most abundant among the pebbles, which range in size up to a diameter of (rarely) 1 foot. 



The only identifi.able fossil invertebrate yet obtained from the Ignacio beds was found 

 on a remnant capping Overlook Point, one of the hdls of Mountain View Crest, in the Needle 

 Mountains quadrangle, south of Needle Creek. Specimens of this fossil were found scattered 

 through a hard, dark quartzite above the middle of the formation, which was there 110 feet 

 thick. Mr. Walcott identifies this shell as an Obolus but is unable, from the material at hand, 

 to determine its species. Three forms that are much like this are Oholus matinalis, 0. tetonensis, 

 and 0. loperi, the latter occurring in the passage beds between the Cambrian and the Ordo- 

 vician in the Crested Butte quadrangle, Colorado. Since the Cambrian is known in Colorado 

 only in thin representatives of the upper division, it seems best to assume for the present that 

 the Ignacio quartzite also belongs in that series. 



A common apparent fossil of the Ignacio is thought by Mr. Walcott to be Cruziana, a 

 problematic plant remain. 



J 15. MISSOURI AND NOBTHERN ARKANSAS. 



The "Magnesian Limestone series" of Missouri is thus described in general 

 terms by Broadhead : ^^ 



The chief rocks of the Ozark Plateau include a series of magnesian limestones and sand- 

 stones amounting to 1,000 feet in thicloiess. In 1891 I applied to these rocks the name "Ozark 

 series." The lower one-half to two-thirds may be Cambrian, including rocks of the age of the 

 Potsdam of New York. The upper portion may be the equivalent of Dana's Canadian, or 

 the lower part of the Lower Silurian. The evidence shows that the life of the Cambrian sea 

 in Missouri was limited to but a few species. We find a few Orthocerata, a Lituite, a small 

 Orthis, a Pleurotomaria, a Maclurea, two species of OphUeta, two or three species of Straparollus, 

 probably two species of Murchisonia, an Obolella, three or four species of trilobites, while in 

 the Potsdam of Wisconsin and Minnesota several other trilobites may be found. Where the 

 waters were shallow sea worms lived and left their track in irregular windings in the rocks. 

 In the later seas seaweeds abounded, as shown by certain beds of the Second Magnesian lime- 

 stone. That the Ozark series was subject to erosions and slight elevations and depressions is 

 shown by the occasional appearance of a few feet of sandstone at intervals of a few feet to 

 over a hundred. Near the close of the Ozark terrane it is probable that a large area of the 

 plateau was elevated above water. 



The general stratigraphy is stated as follows by the same authority: 



Feet. 



Trenton limestone 



First Magnesian limestone lOp 



First or St. Peters (saccharoidal) sandstone 130 



Second Magnesian limestone 200 



Second sandstone 125 



Third Magnesian limestone 300 



Lower Magnesian limestone and sandstone 300 



1,155 



Several classifications of this series of rocks have been proposed, but the vari- 

 able character and lenticular form of manv of the formations or members have 



