CHARACTERISTIC FOSSILS. 129 



green. It occurs with the rocks described above, and 

 passes, by various gradations, into all. 



Hornblende-schist is considered to include all hornblende 

 rocks, together with greenstone and greenstone- slate, whether 

 these rocks possess a schistose structure or not. 



Quartz-rock is a substance which offers so many variations 

 of mineral character and aspect, that no single description 

 can well define it, and the reader is referred' to Dr. M'Cul- 

 loch's Classification for complete details. 



Crystalline, or primary limestone, also presents consider- 

 able diversity of texture, ranging from the pure or saccharine 

 variety, so termed from its resemblance to loaf-sugar, to a 

 coarse, impure, and cherty limestone.* 



The student should procure a collection of well-defined 

 specimens of these various primary and volcanic rocks. They 

 may be obtained of the parties already mentioned. 



EOSSILIFEROUS ROCKS. These are divided into well- 

 defined groups, as shown incur table of Classification. They 

 should be studied under the different heads of mineral com- 

 position, order of succession, and embedded fossils; for 

 though none of these characteristics is, in all cases, to be 

 relied on by itself, yet when viewed in combination, they 

 will afford complete evidence of the relative date and 

 character of the different stages into which they are sub- 

 divided. Of the proofs thus furnished, that derived from 

 organic remains is the most decisive; rocks, which are 

 extremely dissimilar to each other in mineralogical and 

 lithological characters, having been identified by this evidence 

 alone. Professor Agassiz has discovered fossils of the chalk 

 in deposits of totally different mineral texture and aspect ; 

 while Sir R. Murchison has identified the mountain lime- 

 stone in Russia, by the characteristic shells of that formation 

 in rocks of the colour and appearance of white chalk. 



CHARACTERISTIC FOSSILS. These may be defined to be 

 such organic remains as, being discovered in any given depo- 

 sit, are also found in other localities where deposits of the 

 same age occur. 



MINERAL VEINS. These exist throughout the primary, 

 lower secondary, and in some cases in the tertiary deposits ; 



* For important information on the Metamorphic rocks the reader is 

 referred to Sir Charles Lyell's Elements of Geology. 



