This boss is bounded, except on the south, by the Witwatersrand series, the lower portion of which consists of quartzites and slates and the upper portion of quartzites and conglomerates.
The western portion of Kotelnyi is built up of Middle Devonian limestones and slates, folded the same way, of which the fossil fauna is similar to that of the Urals.
Triassic slates appear in the south-east.
The baptismal fonts date from the 12th century, and the curious spire in the form of an elongated pumpkin and covered with slates gives a fantastic and original appearance to the whole edifice.
They are rarely metamorphosed to the point of recrystallization, though locally shales are altered to roofing slates, sandstones are indurated, limestones slightly marblized, and coals, originally bituminous, are changed to anthracite in northern Pennsylvania, and to graphite in Rhode Island.