walcott.J NOMENCLATURE. 245 



ST. CROIX. 



The name St. Croix was proposed by Prof N". H. Winchell in 1873 

 for the fossiliferous sandstone beneath the Lower Magnesian limestone 

 of the Mississippi Valley. The reason given is that the sandstone iden- 

 tified by Dr. D. D. Owen as the Potsdam sandstone is unconformably 

 superjacent to a series of quartzites in Wisconsin and Minnesota that 

 are the true equivalents of the Potsdam sandstone of New York. 1 



The presence of the same fauna in the typical Potsdam sandstone on 

 the northern side of the Adirondack Mountains as that found in the 

 upper sandstone of St. Croix Falls negatives this view and correlates 

 the sandstones that are unconformably superjacent to the lower quartz- 

 ites with the typical Potsdam, and refers the lower quartzites to the 

 Algonkian. The name St. Croix, however, is retained for the fossiliferous 

 Cambrian sandstones of the upper Mississippi Valley in Wisconsin, 

 Iowa, and Minnesota. 



MADISON. 



The name Madison sandstone was proposed by Prof. R/D. Irving, 2 in 

 1877, for a band of sandstone consisting, in large part, of nearly pure 

 white quartz sand, with a thickness of 35 to 50 feet, and lying between 

 the base of the Magnesian limestone of the Lower Silurian (Ordovician) 

 and the Mendota limestone in Wisconsin. 



MENDOTA. 



This name was proposed by Prof. R. D. Irving, in 1877, for a cal- 

 careous stratum near the upper portion of the " Potsdam v formation of 

 Wisconsin. It is formed of alternating saudy and calcareous material 

 that, in the upper portion, is almost a well marked and very persistent 

 yellow limestone layer, with a thickness of 30 feet. 3 



TONTO. 



.This name was applied by Mr. G. K. Gilbert, in 1874, 4 to a series of 

 sandstone and shale between the subjacent granite and the superjacent 

 massive limestones that occur in the section at the mouth of the Grand 

 Caliou of the Colorado River, Arizona. He considered the formation 

 of Primordial age, and it has since been found to contain an Upper 

 Cambrian fauna. It was more fully described by him in the following 

 year. 5 



1 General sketch of the geology of Minnesota. Geol. and Nat. Hist. Surrey, Minnesota, 1st Ann. 

 Rep. for 1872, 1873, pp. 70-72. 



2 The Lower Silurian rocks. (Geology of central Wisconsin.) Geol. Wisconsin, survey of 1873-1879, 

 vol. 2, 1877, p. 525. 



•Ibid., 525. 



4 On the age of the Tonto sandstone. (Abstract.) Washington Phil. Soc. Bull., vol. 1, 1874, p. 109. 



« Explorations and surveys west of the 100th Merid., vol. 3, 1875, p. 163 ; pp. 521-522. 



