THE LOWER FOBMS OF LIFE. 49 



To those who are unacquainted with geology, I may remark briefly 

 that, although the entire solid crust of the earth has been calculated 

 by Mr. Hopkins to be not less than 800 or 1000 miles thick, there 

 are only about nineteen which are known to geologists as sedimen- 

 tary-fossiliferous rocks that is to say, rocks which have been 

 deposited from water in different periods of the world's history, and 

 containing preserved within their beds the remains of some of the 

 living things which existed in those waters. 



Now this nineteen miles of sedimentary rocks may be broadly 

 divided by the character of their organic remains, so as to represent 

 four nearly equal divisions. The first of these from above down- 

 wards which is, of course, in order of age, the fourth period 

 comprises 1, all recent formations, including those strata called by 

 some geologists Quaternary ; 2, the Tertiary formations ; 3, the 

 Cretaceous, or Chalk group ; 4, the Jurassic, so called because they 

 are typically developed in the Jura Mountains ; 5, the Triassic, 

 corresponding to the upper new red sandstone, which is readily 

 divisible into three distinct groups hence the name. 



These strata are called Neozoic, because the life which is repre- 

 sented as having existed is "newer" than that which precedes it. 

 Till recently, and even now by many geologists, the term "new " is 

 confined to the recent and tertiary formations, under the term 

 Cainozoic ; while the secondary strata, from the chalk to the lias, 

 are called Mesozoic (middle life). 



The third division comprises 1, the Permian or Magnesian 

 Limestone ; 2, Carboniferous, or Coal measures ; 3, the Devonian ; 

 and these strata are called Upper Paleozoic (old life). 



The second division includes 1, Upper Silurian ; 2, Lower 

 Silurian ; 3, Cambrian. These form the Lower Paleozoic. 



The first division or period, called till recently the Azoic (without 

 life), includes the oldest known sedimentary rocks, " the Laurentian," 

 which are developed largely in Canada and the United States, and 

 are also found in Ireland, Scotland, and Devonshire. The above 

 arrangement will appear more clear from the following table : 



