Page:Walcott Cambrian Geology and Paleontology I.djvu/25

Rh of the same relative horizon is called the Flathead sandstone. As the strata are followed to the northwest, the sandy beds occupy a lower stratigraphic horizon until on Gordon Mountain, at the head of the South Fork of the Flathead River, in Montana, the sandstones are of lower Middle Cambrian age. The Brigham formation should not be confused with the much older Prospect Mountain "quartzite" formation of central Nevada, which is of Lower Cambrian age.

.—Annelid trails and trilobite tracks. Characteristic Middle Cambrian fossils were found in the upper portion of this formation west of Liberty, Bear Lake County, Idaho.

The section exposed in the House Range was first studied by Dr. G. K. Gilbert, who made small collections of fossils from various horizons. These collections were so interesting that I visited the range in 1903. In 1905 I revisited the range, in company with Messrs. F. B. Weeks and L. D. Burling, measured the entire section carefully, and made further large collections of fossils. The section extends from well down in the Lower Cambrian to the base of the Ordovician, and is the best and most complete of the Basin Range sections so far studied. A map will be published with the detailed sections, giving the geographic localities referred to in the nomenclature of the formations of the House Range section.

The following table gives the relative positions and the thickness of the various formations defined in the following pages:

.—Upper portion of the main mass of Notch Peak, House Range, Utah.