90 THE BUILDING OF AN ISLAND. 



in course of fcirmation, such, for instance, as the beach linmestones of our 

 West Indian islands. 



The arrangement adopted by the English geologists has, with some modi- 

 fications, been very largely accepted by geologists in general. 



The following table of the English systems is taken from Marr's Intro- 

 duction to Geology, explanations being here added after the names, as the 

 most concise way of giving them : 



Systems. 



f Recent (Quaternary of some authors) 



I Pleistocene (Meaning most recent period) 



Tertiary ] Pliocene (Meaning more recent period) 



j Miocene (Meaning less recent period) 



L Eocene (Meaning Dawn of the recent period) 



r Cretaceous (Chalk system) 



Secondary ; Jurassic (Named from the Jura Mountains) 



L New Red Sandstone 



r Carboniferous (Coal-bearing system) 



I Devonian (From County Devon Old Red Sandstone in North of England) 



I Silurian (From the Silures, a British Tribe formerly occupying South Wales) 



Primary ' Ordovician (From the Ordovices, a British Tribe formerly occupying Northwest 

 j Wales) 



I Cambrian (Cambria = Wales) 



L Precambrian (Before the Cambrian) 



By way of further explanation, it may be added that the lower three 

 systems of the Tertiary division were named by the great English geologist, 

 Sir Charles Lvell, who called the lowest Eocene (dawn of the recent period) 

 because it was found that only 31^ per cent, of the fossil shells taken from it 

 belonged to existing species. The proportion of existing species increased 

 through the younger rocks, till in what Sir Charles called the Newer Pliocene 

 all the shells belonged to existing species, the formation containing, however, 

 the bones of certain quadrupeds which had become extinct. Later geologists 

 have substituted the name Pleistocene for Newer Pliocene and have added 

 Oligocene as a system to come between Eocene and Miocene, the name im- 

 plying that the system contains the remains of a few recent species. 



The reader who follows up the subject will find considerable difTerences in 

 naming the systems as well as the great divisions, and some differences also in 

 arrangement. Particularly he will find that the simple names of Primary, 

 Secondary and Tertiary have been replaced by names of life-periods ; and he 

 will find that the New Red Sandstone has been divided into Trias (so named 

 because including three different sets of strata) and Permian (so named from 

 the province of Perm in Russia, where it is conspicuous) and that the Trias 

 has been retained in the middle life-period, while the Permian is placed in the 

 ancient life-period. 



The following is the arrangement adopted by the United States Geological 

 Survey : 



