the building of an island. 91 



Period. 



r Pleistocene 

 Cenozoic j Neocene (Pliocene 



(Recent life-period) i (new recent) (Miocene 



L Eocene, including Oligocene 



Mesozoic < Cretaceous (Jurassic 



(Middle life-period) ( Juratrias < Triassic 



r- Carboniferous, including Permian 

 Palajozoic ] Devonian 



Ancient life-period I Silurian, including Ordovician 



L Cambrian 



Algonkian 

 Archaean 



It will be seen that the above (differs little from Dr. Marr's table. The 

 term Precambrian, though here replaced by part of the Archican (ancient) and 

 the Algonkian (from the territory of the Algonquin Indians) is still sometimes 

 used by the American geologists to include very ancient stratified rocks 

 without fossils, and rocks such as those gneisses the original stratification 

 of which is doubtful. The term "Recent" in Dr. Marr's table, omitted in 

 the above, appears to be unnecessary, as the name of a system, when we 

 have " Pleistocene." 



It would far exceed the purpose of this chapter to try to describe, in how- 

 ever brief a fashion, the characteristic rocks and fossils of each of the above 

 systems. By way of illustration, however, we may take the English Carbon- 

 iferous system. As the name implies, it contains coal beds, but they are by 

 no means all of the system ; they form, indeed, a very small part of it. The 

 most conspicuous rock of the lower part of the system is the Mountain Lime- 

 stone, a hard blue-grey stone forming beds of two or three thousand feet in 

 total thickness, some of them containing mostly shells, some mainly corals, 

 others again full of stems of feather-stars (or sea-lilies), the encrinite limestone 

 mentioned in the preceding chapter. Some shales (hardened clays) are found 

 both above and below these limestones; then as we ascend, we come to great 

 beds of coarse sandstones known as Mill-stone Grit, having a total thickness 

 as great as that of the Mountain Limestone, and lastly we arrive at the set 

 called the Coal Measures. These "Coal Measures" are made up of an 

 immense thickness of shales and sandstones, clays and limestones, and contain 

 much concretionary iron ore, the coal beds themselves, though by far the most 

 important, forming only a small proportion of them. The coal beds, or seams, 

 as they are commonly called, vary in thickness from 20 feet or more down to 

 beds so thin as not to pay for the working. A coal bed commonly rests on a 

 bed of clay called the underclay and is often covered by a bed of dark shale 

 known as roofing shale. In the underclay the roots of the trees and smaller 

 plants from which the coal was formed can sometimes be seen, and occasionally 

 parts of the stems of the trees are found attached to the roots. These included 



