The term is a relic of that early period in the history of stratigraphy when each group of strata was supposed to be distinguished by some peculiar lithological character.
There occur also below the Lias on some parts of the west coast unfossiliferous red sandstones, conglomerates and breccias, presenting lithological resemblance to the Rhaetic group of England.
On the east side in North Glen Sannox Burn, they are associated with cherts, grits and dark schists with pillowy lavas, tuffs and agglomerates which, on lithological grounds, have been regarded as probably of the same age as the Arenig cherts and volcanic rocks in the south of Scotland.
The lithological characters of the Cambrian rocks possess a remarkable uniformity in all quarters of the globe.
The Severn down to Worcester, draining 1,256,000 acres of generally flatter land largely of the same lithological character, gave in the dry season from the 1st of July 1887 to the 30th of June 1888 a loss of 17.93 in.