Page:A Treatise on Geology, volume 1.djvu/70

limited differences are often more important in theory (as well as in practical applications) than all the general resemblances. Assuming that the British islands form such a natural district, we shall be able to present a satisfactory general table or section of the series of strata which here compose the crust of the globe, placed in the order of their succession downwards, from the surface of the most recent aqueous deposits.

Table of British Deposits.

Superficial Accumulations.
Depositions from springs, rivers, glaciers, lakes, the sea, under ordinary circumstances.
Depositions from some of these agents under extraordinary circumstances.
Stratified Rocks.
Tertiary or Cainozoic Strata.
Names of formations.	Thickness in yards.		Subdivisions or groups.	Nature of the deposits.
Crag.	16	$\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$	Upper or mammaliferous crag.	Marine and estuary shells, pebbles, bones, sand, &c.
			Middle or red crag.	Marine shells, pebbles, bones, sand, &c.
			Lower or coralline crag.	Marine shells and corals in sand, or coarse limestone.
Freshwater marks.	33	$\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$	Upper freshwater.	Marly limestone and clays.
			Estuary beds.	Marine or estuary clays, marls, &c.
			Lower freshwater.	Marly limestone and clays.
London Clay.	200 to 600	$\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$	London clay (upper).	Clay with septaria, &c.
			Sand and clays.	Coloured sands, clays.
			London clay (lower).	Clay with septaria, &c.
			Plastic clay.	Variegated sands, clays, lignite, &c.
Secondary or Mesozoic Strata.
Cretaceous System.
Chalk.	200	$\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$	Upper chalk.	Soft chalk, with flints in layers.
			Lower chalk.	Harder chalk.
			Chalk marl.	Soft argillaceous chalk.
Green Sand.	160	$\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$	Upper green sand.	Green sands.
			Gault.	Blue marl or clay.
			Lower green sand.	Ferruginous, brown, or green sand, with local deposits of limestone.