Page:Canadian Alpine Journal I, 2.djvu/73

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233
Mt. Stephen Rocks and Fossils

she could to open up and make accessible the great storehouse of fossils contained in the shales. Nearly every fragment of shale found on the slopes from 2000 to 2600 feet above Field has fossils upon it; not only fragments, but usually entire specimens of trilobites. The fossil bed thins out rapidly to the northeast and southwest. It is in fact a lens-shaped formation, thinning out from the center in all directions. The shales were originally a sandy mud that was slowly deposited as thin layers in quiet water. For some unknown reason, the trilobites died by thousands and were buried by the successive layers of mud. Small marine shells occur quite abundantly in some of the layers along with the trilobites and smaller fossils of various kinds. The largest and most abundant trilobite is called Ogygopsis klotzi, and from it the name Ogygopsis shale is given to the band or lens of siliceous shale in which the trilobite occurs.

The Stephen formation, with the Ogygopsis shale, forms the dark, bluish-gray band that extends across the north face of the mountain just above the shoulder, over the railroad tunnel. Another dark band of limestone, 150 feet thick, that shows in all photographs of Mount Stephen from the north, is 650 feet higher up, the interval being occupied by massive beds of gray siliceous limestone. A few fragments of Middle Cambrian fossils occur in the dark, bluish-gray limestone. Above the dark band, massive beds of gray, sandy limestone rise tier above tier for 2700 feet to the summit of the mountain. This great series is called the Eldon formation, from Eldon, north of which, in the slopes of Castle Mountain, it has a fine development.

Southwest of Mount Stephen the layers of rock are broken and bent to the southwest and west until they pass beneath Mount Dennis. All belong to the Cambrian period. A few fossils occur in the amphitheatre east of Mount Dennis, but the best collecting