The region is underlain by (?)Hadrynian to Paleozoic Snowshoe Group rocks. The Snowshoe Group is an assemblage of dominantly metasedimentary rocks within the Barkerville Terrane of south-central British Columbia. The metasedimentary rocks consist primarily of marble, quartzite and phyllite. In the Yanks Peak area these rocks comprise the Keithley and Harveys Ridge successions, but further to the east they remain undifferentiated. Metamorphism of the region varies from chlorite to sillimanite and higher grade. Gold-bearing quartz veins occur only in greenschist facies rocks.
The Cornish Ledges veins outcrop at an elevation of 1829 metres on a gentle slope that falls off eastward into the divide area between the heads of French Snowshoe Creek and a branch of Cunningham Creek, about 22 kilometres south of Barkerville. The veins are about 1829 metres north of the Jim adit (093A 037).
The Cornish Ledges consist of five main veins and two smaller veins outcropping in an area about 61 metres wide and 91 metres long. The veins are hosted in grey, coarse quartzite and grit of the lower Snowshoe Group. These rocks strike about 330 degrees and dip 30 to 40 degrees northeast, forming a thin shell over a northwesterly plunging anticlinal septum of Midas Formation (Snowshoe Group) black silty quartzite which outcrops in a small area to the southeast.
The veins strike 300 to 310 degrees, one dips 80 to 85 degrees southwest, and all the others except one dip 80 degrees northeast. This one, on which the Cornish Ledge opening was made, dips 80 degrees northeast at its southeast end and 70 degrees southwest at its northwest end. The veins cut across both the strike and dip of the beds elsewhere and occupy a set of fractures differing in strike from any that have been observed.
The veins have a maximum width of 1.2 metres and a greatest exposed length of 39 metres. The quartz for the most part is massive and unfractured, although the free walls of the Cornish Ledge indicate a small amount of right-hand post-mineral movement. The quartz is very sparsely mineralized with galena and pyrite; galena appears to be the more abundant. Although gold is said to have been mortared out by Cornish miners, no visible gold was seen, nor has any been reported in recent times. In 1954, a selected sample of quartz containing about 50 per cent galena and some pyrite assayed 0.28 gram per tonne gold and 402.6 grams per tonne silver (Bulletin 34).
Quartz veins are also exposed in a series of opencuts about 122 metres north of the Cornish Ledges showing. Seven opencuts expose quartz for a length of 122 metres. Continuity of the quartz between individual exposures is not definitely established but the presence of a vein striking about 335 degrees is probable. The vein appears to lie along the contact of Snowshoe Group grits and quartzites on the southwest and Midas Formation black silty quartzite on the northeast and to cut steeply across the dip of the beds, but this relationship is not definitely established. The vein reaches a maximum width of 4.3 metres in one opencut and in five others is 1.8 to 3 metres wide. The quartz is sparsely mineralized with pyrite and galena; in one opencut some lacy-textured marcasite is present.
In 1954, the Cornish Ledges were located on the Scott No. 5 claim, one of a group of claims held by S. Allison, of Cache Creek, and H. Matte and N. Scott, of Wells. The history was that these veins were worked in the very early days by a group of Cornish miners who mortared gold from the outcrops; work appears to have been done on only one vein. The old working consists of an opening about 7.6 metres in length and possibly averaging 0.9 metre in depth. Until the autumn of 1951, S. Allison and his partners, H. Matte and N. Scott, held four groups (the Allison, Matte, Scott, and Louvelette), totalling twenty-five recorded claims lying to the north of the Cornish Ledges. An examination of some of the veins on them indicated that bands of Midas and Snowshoe rocks which were mapped to the south extend northwestward across the claims and most veins appear to be in Snowshoe quartzites. The veins appear to fall into two groups, including those striking from 050 to 070 degrees, and those striking 090 to 120 degrees. All are more or less vertical. The quartz is mineralized with pyrite or galena, or both. In no instance is the amount of sulphide mineralization very large, and for the most part it is considerably less than 5 per cent.
