122 Lecture on the Geology of [No. 9, new series. 



With reference to the fossils it is not so easy to give an expla- 

 nation in few words ; but some idea may be formed from the well- 

 ascertained fact, that certain animals have existed for a certain 

 period, and then wholly disappeared and been succeeded by other 

 animals of different species, which, in turn, have again given place 

 to others. 



So, as Sir Charles Lyell truly says, " a series of sedimentary 

 formations is like volumes of history, in which each writer has 

 recorded the annals of his own times, and then laid down the 

 book with the last written page uppermost." And the organic 

 remains are, as Dr. Mantell beautifully expresses it, the " coins of 

 Creation," which give us the means of tracing the history of the 

 development of the organic kingdoms. 



Thus, by superposition and by their organic remains, the aqueous 

 rocks are divided into groups forming, in reference to their age, 

 what is termed an " ascending series," or beginning with the oldest 

 in the following manner : — 



1. Primary formations or periods. 



2. Secondary „ „ 



3. Tertiary „ „ 



4. Quartary ,, „ 



In reference to the word " quartary," I may explain that, al- 

 though it is not an English word, I take the liberty to use it in 

 the sense of " post tertiary," as following the analogy of the other 

 terms. 



Each of those formations is again divided into numerous minor 

 systems, on which I have no time to enter. 



The fourth and last great division of rocks are the volcanic — as 

 Trachyte, Basalt, Breccia, and Tuff — all produced by supra- 

 marine or submarine volcanic eruption. It is ascertained that the 

 earliest true volcanic eruptions have occurred subsequently to the 

 secondary period, commencing in the Tertiary, and continuing to 

 the present time ; and it is a marked difference between the older 

 and the more recent eruptions. 



I have prepared a diagram which will serve to impress these 

 first principles upon your memory, and so enable you to follow 

 me in the account I have to give : — 



