i68 ELEMENTS OF SCIENCE 



Each of these three great groups of rocks is made up 

 of a certain number of subordinate groups of strata or 

 formations grouped in systems. Thus the Palaeozoic, or 

 Primary rocks, are made up of the Eozoic-archcean 

 (including the Laurentian formation), Cambrian, Ordo- 

 vician, Silurian, Devonian, Old Red Sandstone, Car- 

 boniferous and Permian systems. The Laurentian rocks 

 are very largely developed in Canada, and are some 

 30,000 feet in thickness. The Cambrian rocks are 

 from 15,000 to 20,000 feet thick, and are well seen 

 in the Longmynds of Shropshire, and near Bangor, 

 Harlech, and St. Davids in Wales. The Silurian 

 strata (sandstones, clays, limestones, and igneous rocks) 

 are of very great thickness, and form a large part of 

 Wales, the lake district of England, Southern Scotland, 

 and some parts of Ireland. The Devonian system is 

 exemplified in Devon and Cornwall, and the Old Bed 

 Sandstone rocks of Ireland, Scotland, and Wales. The 

 Carboniferous system includes the Carboniferous 

 limestone and the Coal measures. The latter consists of 

 seams of coal and layers of sandstone and slate; such 

 alternations indicating oscillation of level. The Permian 

 system is of moderate thickness, and mainly consists 

 of magnesian limestone associated with many slates and 

 beds of conglomerate. In England it is chiefly found 

 skirting the coal-fields from Durham to Derbyshire. The 

 Mesozoic or Secondary rocks, are made up of Triassic, 

 Jurassic, Neocomian, and Cretaceous systems ; the first 

 (Trias} which includes strata known as the " New Red 

 Sandstone " extends in England from Devon to York- 

 shire, and is largely developed in Cheshire. The Jurassic 

 rocks contain all formations from the Lias and the Oolite. 

 to the Purbeck beds. The Lias extends from Lyme Regis, 

 obliquely north-east to Whitby. The Oolite also extends 



