Sutherland Brown (1957, p. 29) suggested two envi- ronments of deposition for the Midas Formation. For the Vic Sandstone he proposed a continuation of the shal- low marine environment represented by Yanks Peak For- mation quartzite. Above the Vic, deposition would have occurred in deeper water with a restricted circulation. However, this restricted deep-water condition is invoked to account for extensive deposits of black slate and phyllite now included in the Harveys Ridge succession of the Barkerville Terrane. Young (in Campbell et al., 1973) postulated a calm, restricted marine deposition based on dark fine clastics and evidence of organic material. He suggested deposi- tion on a deep shelf or basin because the Midas Forma- tion graded westward into thick black shales. The Midas Formation does not consist of thick black shale in the map area and exposures farther west are not Midas For- mation, as mentioned above. Mansy (1970) used similar arguments for restricted marine deposition but also stated that the fine grained quartzites have grain size distribu- tions which are characteristic of fluviatile origins. Deposition associated with restrictive marine condi- tions may not have been continuous in the west, but rather involved isolated areas on the margins of the restricted basin, outlined by Young, to the northeast. Midas Formation deposition may have continued in the environment established during Yanks Peak Formation time until after Vic Sandstone time, as suggested by Sutherland Brown (1957). Then restrictive conditions may have begun in the east with some areas of the west also affected. Another possibility is that the lower Midas For- mation, including the Vic is synchronous with the upper Yanks Peak Formation farther east. The onset of Midas Formation deposited under restrictive conditions would be synchronous in both the east and west. Western depo- sition during this time would be into quiet water but both restricted and nonrestricted environments. Age and correlation. Young (1969, in Campbell et al., 1973) assigned the Midas Formation to early Lower Cam- brian from evidence of abundant trace fossils, from the gradation into early Lower Cambrian Mural Formation, and from the correlation of Midas Formation with upper- most McNaughton Formation which contains cruziana- like trace fossils. No new evidence has been obtained and this age assignment is accepted. Mural Formation The Mural Formation consists mainly of limestone and lesser amounts of dolostone. To the east in Cariboo Mountains it also contains shale. Isolated, unfossilifer- ous, exposures of Mural Formation limestone can be con- fused with limestone of the Yankee Belle, Cunningham and Isaac formations of the Cariboo Terrane and with most limestone units of the Snowshoe Group of the Barkerville Terrane. It is distinguished by its stratigraphic position, occurrence of archaeocyathid fossils and from its contact relations with bounding units. 20 The Mural Formation is widespread, occurring throughout the Cariboo Terrane within the area and is correlated eastward into the Rocky Mountains where it was first named. Campbell et al. (1973) described the for- mation from the Rocky and Cariboo mountains and pre- sented a reference section at the head of Dome Creek. The thickness increases westward from 150 m near the Rocky Mountain Trench in Cariboo Mountains to 730 m at Turks Nose Mountain which borders the eastern edge of the map area (Campbell et al., 1973, p. 51). Thickness variations within the map area are not well known, because the Mural Formation is in most places wholly or partly bevelled below an unconformity at the base of the Black Stuart Group, and because of struc- tural complications. It is 150 m thick on the south slope of Kimball Ridge where it is stratigraphically bounded by the underlying Midas and overlying Dome Creek for- mations; much thinner than at Turks Nose Mountain 15 km to the northwest. It is 250 m thick on Anderson Ridge where it is unconformably overlain by the Black Stuart Group. The lower contact with the Midas Formation is con- formable and mainly sharp. The uppermost 2 to 6 m of Midas Formation commonly contain | to 2 cm thick lenses of limestone which thicken upsection to 2 to 8 cm beds directly below the massive limestone of the Mural Formation. The Midas Formation pelite becomes more intensely green near the contact and at the headwaters of Roundtop Creek it is interbedded with purple pelites. The character of transition from Midas to Mural Formation can be used to differentiate Mural from Cunningham Formation. The Isaac-Cunningham transition differs from the Midas-Mural transition in two ways: 1) the Isaac is dis- tinct from the Midas Formation because it is predomi- nantly black pelite and calcareous pelite whereas the Midas Formation is mostly green and grey pelite inter- bedded with grey or light grey siltite and quartzite; and 2) the Isaac-Cunningham transition is gradational over approximately 100 m, with an upward increase in brown weathering dark grey limestone grading to the grey weathering grey limestone of the Cunningham Forma- tion, in contrast to the rapid transition of 2 to 6 m between Midas Formation, which may have thin carbo- nate lenses developed adjacent to the contact, and the massive to 10 cm bedded grey limestone of the Mural Formation. Limestone of the Mural Formation is grey to dark grey where it is relatively unaltered and dark grey to white where altered. It weathers mainly grey but can be buff, white or orange. It generally consists of 0.05 to 0.5 mm metamorphic crystals of calcite with minor amounts of quartz and heavy mineral silt. Bedding is preserved locally (Fig. 10) with features such as oolites and archaeocya- thid and trilobite fragments in limestone as on Kimball Ridge. Sedimentological detail is usually lost due to recrystallization, being replaced by 0.5 to 2 cm colour- defined laminations which appear in most cases to parallel