CURRENT PRE-CAMBRIAN LITERATURE 847 
Park, and are best exposed on the southern slope of Mount Sheridan, 
from which the formation has been called the Sheridan quartzite. 
The Sheridan quartzite formation comprises sandstones and slates, 
which contain no fossils. Unconformably overlying the Sheridan 
quartzite is the Ellis (Juratrias) limestone. The assignment of the 
formation to the Algonkian is based largely on the fact that similar 
rocks are unknown in the Paleozoic series, and on the fact that no 
sedimentary rocks older than these quartzites are exposed in this dis- 
trict. 
Hague,' in a discussion of the age of the igneous rocks of the Wel- 
lowstone National Park, mentions the occurrence of rocks of Archean 
age in the surrounding mountain ranges. The Tetons, bordering the 
park to the south, consist mainly of an Archean mass, which towers 
high above all later rock formations. In the Absaroka range, stretch- 
ing along the entire east side of the park, and formed mainly of igne- 
ous rocks, granite and schists are exposed at the northern end.) Die 
Snowy range, which shuts in the park to the north, is largely made up 
of Archean schists, gneisses, and granites, associated with the more 
recent outbursts of lava. In the Gallatin range, on the west, a body 
of crumpled gneisses and schists forms the nucleus of the mass. The 
Archean masses formed either a part of a broad continental mass, or a 
group of closely related islands. Resting unconformably upon the 
Archean are great thicknesses of Paleozoic and Mesozoic rocks. 
Eldridge? gives an account of a geological reconnaissance across 
Idaho, on a northeast line through Boisé and Salmon City. Rocks are 
found which are provisionally referred to the Archean and Algonkian. 
To the Archean are referred granite and gneiss, which have their 
greatest development in the mountains of the western part of the state, 
but which are also widely exposed elsewhere. In places in the granite 
and gneiss are included bands of calcareo-micaceous or quartzitic 
slates, and in these cases the reference of the rocks to the Archean, 
instead of the Algonkian, is questionable. To the Algonkian is pro- 
visionally assigned the great series of micaceous, quartzitic, and 
chloritic schists of eastern Idaho. The reference is based merely upon 
«The age of the igneous rocks of the Yellowstone National Park, by ARNOLD 
Hacur. Am. Jour. Sci., 4th ser., Vol. I, pp. 445-457, 1896. 
2A geological reconnaissance across Idaho, by GrorGE H. ELDRIDGE. Six- 
teenth Ann. Rept. U. S. Geol. Surv., Part IT, 1895, pp. 217-276. 
